Improve top-level exception handling

Summary:
Fixes T6692. Addresses two main issues:

  - The write guard would sometimes not get disposed of on exception pathways, generating an unnecessary secondary error which was just a symptom of the original root error.
    - This was generally confusing and reduced the quality of reports we received because users would report the symptomatic error sometimes instead of the real error.
    - Instead, reflow the handling so that we always dispose of the write guard if we create one.
  - If we missed the Controller-level error page generation (normally, a nice page with full CSS, etc), we'd jump straight to Startup-level error page generation (very basic plain text).
    - A large class of errors occur too early or too late to be handled by Controller-level pages, but many of these errors are not fundamental, and the plain text page is excessively severe.
    - Provide a mid-level simple HTML error page for errors which can't get full CSS, but also aren't so fundamental that we have no recourse but plain text.

Test Plan:
Mid-level errors now produce an intentional-looking error page:

{F259885}

Verified that setup errors still render properly.

@chad, feel free to tweak the exception page -- I just did a rough pass on it. Like the setup error stuff, it doesn't have Celerity, so we can't use `{$colors}` and no other CSS will be loaded.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley, chad

Maniphest Tasks: T6692

Differential Revision: https://secure.phabricator.com/D11126

src/__phutil_library_map__.php

@@ -161,12 +161,14 @@ phutil_register_library_map(array(
     'AphrontResponse' => 'aphront/response/AphrontResponse.php',
     'AphrontSideNavFilterView' => 'view/layout/AphrontSideNavFilterView.php',
     'AphrontStackTraceView' => 'view/widget/AphrontStackTraceView.php',
+    'AphrontStandaloneHTMLResponse' => 'aphront/response/AphrontStandaloneHTMLResponse.php',
     'AphrontTableView' => 'view/control/AphrontTableView.php',
     'AphrontTagView' => 'view/AphrontTagView.php',
     'AphrontTokenizerTemplateView' => 'view/control/AphrontTokenizerTemplateView.php',
     'AphrontTwoColumnView' => 'view/layout/AphrontTwoColumnView.php',
     'AphrontTypeaheadTemplateView' => 'view/control/AphrontTypeaheadTemplateView.php',
     'AphrontURIMapper' => 'aphront/AphrontURIMapper.php',
+    'AphrontUnhandledExceptionResponse' => 'aphront/response/AphrontUnhandledExceptionResponse.php',
     'AphrontUsageException' => 'aphront/exception/AphrontUsageException.php',
     'AphrontView' => 'view/AphrontView.php',
     'AphrontWebpageResponse' => 'aphront/response/AphrontWebpageResponse.php',
@@ -3217,11 +3219,13 @@ phutil_register_library_map(array(
     'AphrontRequestTestCase' => 'PhabricatorTestCase',
     'AphrontSideNavFilterView' => 'AphrontView',
     'AphrontStackTraceView' => 'AphrontView',
+    'AphrontStandaloneHTMLResponse' => 'AphrontHTMLResponse',
     'AphrontTableView' => 'AphrontView',
     'AphrontTagView' => 'AphrontView',
     'AphrontTokenizerTemplateView' => 'AphrontView',
     'AphrontTwoColumnView' => 'AphrontView',
     'AphrontTypeaheadTemplateView' => 'AphrontView',
+    'AphrontUnhandledExceptionResponse' => 'AphrontStandaloneHTMLResponse',
     'AphrontUsageException' => 'AphrontException',
     'AphrontView' => array(
       'Phobject',
@@ -4669,7 +4673,7 @@ phutil_register_library_map(array(
       'PhabricatorMarkupInterface',
     ),
     'PhabricatorConfigProxySource' => 'PhabricatorConfigSource',
-    'PhabricatorConfigResponse' => 'AphrontHTMLResponse',
+    'PhabricatorConfigResponse' => 'AphrontStandaloneHTMLResponse',
     'PhabricatorConfigSchemaQuery' => 'Phobject',
     'PhabricatorConfigSchemaSpec' => 'Phobject',
     'PhabricatorConfigServerSchema' => 'PhabricatorConfigStorageSchema',