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, tuomaspelkonen, codeblock
CC: aran, epriestley
Differential Revision: 823