R1:966778c2bdfb

Improve unhandled exception dialogs

Summary:
Make the unhandled exception dialogs slightly more useful:

- Make them easier to read.
- Link to files from Phabricator libraries.
- Don't show traces by default.
- Show traces in development mode.
- Rename button from "Cancel" to "Close" and only show it for Ajax.

Test Plan: Rigged DirectoryHomeController to throw, loaded home page. Changed
stack trace setting in config. Clicked some files in the trace.
Reviewed By: aran
Reviewers: aran, jungejason…