Improve error messages when hitting PHP file upload issues

Summary: See T429. When you hit certain errors, you get less-than-helpful
messages like "upload error 3". Instead, produce human-readable errors.
Test Plan: Simulated errors, verified user receives decent error messages.
Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran, startupguy
CC: aran
Differential Revision: 816

src/applications/files/storage/file/PhabricatorFile.php

@@ -48,7 +48,7 @@ class PhabricatorFile extends PhabricatorFileDAO {
 
     $err = idx($spec, 'error');
     if ($err) {
-      throw new Exception("File upload failed with error '{$err}'.");
+      throw new PhabricatorFileUploadException($err);
     }
 
     $tmp_name = idx($spec, 'tmp_name');