diff --git a/extensions/templates/extensions/submit.html b/extensions/templates/extensions/submit.html
index d77153f0..e896a7bd 100644
--- a/extensions/templates/extensions/submit.html
+++ b/extensions/templates/extensions/submit.html
@@ -59,6 +59,13 @@
                   {% include "common/components/field.html" with field=form.source label='File' classes="js-agree-with-terms-trigger js-submit-form-file-input" %}
                 </div>
               </div>
+
+              {% if form.errors %}
+                <blockquote class="mb-0"><i class="i-info"></i>
+                  Build your extension using the <a class="text-underline" href="https://docs.blender.org/manual/en/dev/advanced/extensions/command_line_arguments.html#subcommand-build">command-line tool</a> to catch and prevent most of these errors.
+                </blockquote>
+              {% endif %}
+
               <div class="row">
                 <div class="col mx-4 mt-4">
                   {% include "common/components/field.html" with field=form.agreed_with_terms classes="js-agree-with-terms-trigger js-agree-with-terms-checkbox" %}
diff --git a/extensions/tests/files/invalid-theme-multiple-xmls-macosx.zip b/extensions/tests/files/invalid-theme-multiple-xmls-macosx.zip
new file mode 100644
index 00000000..2e8ccee1
Binary files /dev/null and b/extensions/tests/files/invalid-theme-multiple-xmls-macosx.zip differ
diff --git a/extensions/tests/test_submit.py b/extensions/tests/test_submit.py
index dd4d16b3..687726bd 100644
--- a/extensions/tests/test_submit.py
+++ b/extensions/tests/test_submit.py
@@ -109,19 +109,20 @@ EXPECTED_VALIDATION_ERRORS = {
         ],
     },
     'invalid-addon-no-init.zip': {
-        'source': ['An add-on should have an __init__.py file.'],
+        'source': ['Add-on file missing: <strong>__init__.py</strong>.'],
     },
     'invalid-addon-dir-no-init.zip': {
-        'source': ['An add-on should have an __init__.py file.'],
+        'source': ['Add-on file missing: <strong>__init__.py</strong>.'],
     },
     'invalid-no-manifest.zip': {
         'source': ['The manifest file is missing.'],
     },
-    'invalid-manifest-toml.zip': {'source': ['Could not parse the manifest file.']},
-    'invalid-theme-multiple-xmls.zip': {'source': ['A theme should have exactly one XML file.']},
+    'invalid-manifest-toml.zip': {'source': ['Manifest file contains invalid code.']},
+    'invalid-theme-multiple-xmls.zip': {'source': ['Themes can only contain <strong>one XML file</strong>.']},
+    'invalid-theme-multiple-xmls-macosx.zip': {'source': ['Archive contains forbidden files or directories: __MACOSX/']},
     'invalid-missing-wheels.zip': {
         'source': [
-            'A declared wheel is missing in the zip file, expected path: addon/wheels/test-wheel-whatever.whl'
+            'Python wheel missing: addon/wheels/test-wheel-whatever.whl'
         ]
     },
 }
diff --git a/files/forms.py b/files/forms.py
index 355b0944..f8e2e254 100644
--- a/files/forms.py
+++ b/files/forms.py
@@ -34,13 +34,13 @@ class FileForm(forms.ModelForm):
             'The manifest file should be at the top level of the archive, or one level deep.'
         ),
         # TODO: surface TOML parsing errors?
-        'invalid_manifest_toml': _('Could not parse the manifest file.'),
-        'invalid_missing_init': _('An add-on should have an __init__.py file.'),
-        'missing_or_multiple_theme_xml': _('A theme should have exactly one XML file.'),
+        'invalid_manifest_toml': _('Manifest file contains invalid code.'),
+        'invalid_missing_init': mark_safe(_('Add-on file missing: <strong>__init__.py</strong>.')),
+        'missing_or_multiple_theme_xml': mark_safe(_('Themes can only contain <strong>one XML file</strong>.')),
         'invalid_zip_archive': msg_only_zip_files,
         'missing_manifest_toml': _('The manifest file is missing.'),
-        'missing_wheel': _('A declared wheel is missing in the zip file, expected path: %(path)s'),
-        'forbidden_filepaths': _('Archive contains forbidden files or directories: %(paths)s'),
+        'missing_wheel': _('Python wheel missing: %(path)s'), # TODO <strong>%(path)s</strong>
+        'forbidden_filepaths': _('Archive contains forbidden files or directories: %(paths)s'), #TODO <strong>%(paths)s</strong>
     }
 
     class Meta:
diff --git a/files/utils.py b/files/utils.py
index 4a03d911..1224b865 100644
--- a/files/utils.py
+++ b/files/utils.py
@@ -172,10 +172,21 @@ def find_forbidden_filepaths(file_list):
 def validate_file_list(toml_content, manifest_filepath, file_list):
     """Check the files in in the archive against manifest."""
     error_codes = []
+
+    found_forbidden_filepaths = find_forbidden_filepaths(file_list)
+    if found_forbidden_filepaths:
+        error_codes.append(
+            {
+                'code': 'forbidden_filepaths',
+                'params': {'paths': ', '.join(found_forbidden_filepaths)},
+            }
+        )
     type_slug = toml_content['type']
     if type_slug == 'theme':
         theme_xmls = filter_paths_by_ext(file_list, '.xml')
-        if len(list(theme_xmls)) != 1:
+        # Special treatment for Mac, so the same problem (__MACOSX folders)
+        # doesn't lead to two errors showing.
+        if len(list(theme_xmls)) != 1 and '__MACOSX/' not in found_forbidden_filepaths:
             error_codes.append('missing_or_multiple_theme_xml')
     elif type_slug == 'add-on':
         # __init__.py is expected to be next to the manifest
@@ -192,14 +203,6 @@ def validate_file_list(toml_content, manifest_filepath, file_list):
                 error_codes.append(
                     {'code': 'missing_wheel', 'params': {'path': expected_wheel_path}}
                 )
-    found_forbidden_filepaths = find_forbidden_filepaths(file_list)
-    if found_forbidden_filepaths:
-        error_codes.append(
-            {
-                'code': 'forbidden_filepaths',
-                'params': {'paths': ', '.join(found_forbidden_filepaths)},
-            }
-        )
     return error_codes