Ref T13590. Currently, errors arising from cluster locking (like the "stuck write lock" exception) are not caught and converted into VCS responses on the HTTP VCS workflow.

Catch a broader range of exceptions and convert them into appropriate responses.

• Forced a "stuck write lock" exception, pushed to a Git repository over HTTP.
• Before: generic fatal.
• After: VCS-specific fatal with a useful message in the "X-Phabricator-Message" response header.