PyAPI: don't adjust prefs when an fails to load

Recent addons commit meant that addons would be enabled even if they weren't found.
This would give an error (which is fine), but also remove from preferences.

release/scripts/modules/addon_utils.py

@@ -251,7 +251,7 @@ def _addon_remove(module_name):
             addons.remove(addon)
 
 
-def enable(module_name, default_set=True, persistent=False, handle_error=None):
+def enable(module_name, default_set=False, persistent=False, handle_error=None):
     """
     Enables an addon by name.
 
@@ -308,7 +308,8 @@ def enable(module_name, default_set=True, persistent=False, handle_error=None):
             mod.__addon_enabled__ = False
         except:
             handle_error()
-            _addon_remove(module_name)
+            if default_set:
+                _addon_remove(module_name)
             return None
 
         # 2) try register collected modules
@@ -322,7 +323,8 @@ def enable(module_name, default_set=True, persistent=False, handle_error=None):
                   getattr(mod, "__file__", module_name))
             handle_error()
             del sys.modules[module_name]
-            _addon_remove(module_name)
+            if default_set:
+                _addon_remove(module_name)
             return None
 
     # * OK loaded successfully! *