Improve several Diffusion UI error states

Summary:
Give users better errors and UI:

  - For subpath SVN repositories, default the path to the subdirectory, not to
"/". This makes the home screen useful and things generally less confusing.
  - For unparsed commits, show a more descriptive error message without the
"blah blah" silliness.
  - For paths outside of the subpath parse tree, short circuit into an
appropriate error message.
  - For foreign SVN stub commits (see D892), show an explicit message.

Test Plan: Looked at unparsed commits, subpath repositories, foreign stub
commits, and paths outside of the subpath parse tree. Received sensible error
messages.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 894

src/applications/diffusion/controller/browse/DiffusionBrowseController.php

@@ -75,6 +75,16 @@ class DiffusionBrowseController extends DiffusionController {
           $controller->setDiffusionRequest($drequest);
           return $this->delegateToController($controller);
           break;
+        case DiffusionBrowseQuery::REASON_IS_UNTRACKED_PARENT:
+          $subdir = $drequest->getRepository()->getDetail('svn-subpath');
+          $title = 'Directory Not Tracked';
+          $body =
+            "This repository is configured to track only one subdirectory ".
+            "of the entire repository ('".phutil_escape_html($subdir)."'), ".
+            "but you aren't looking at something in that subdirectory, so no ".
+            "information is available.";
+          $severity = AphrontErrorView::SEVERITY_WARNING;
+          break;
         default:
           throw new Exception("Unknown failure reason!");
       }

src/applications/diffusion/controller/browse/__init__.php

@@ -14,6 +14,7 @@ phutil_require_module('phabricator', 'applications/phid/handle/data');
 phutil_require_module('phabricator', 'view/form/error');
 phutil_require_module('phabricator', 'view/layout/panel');
 
+phutil_require_module('phutil', 'markup');
 phutil_require_module('phutil', 'utils');