Improve handling of gigantic CommandExceptions

Summary:
See T2193. For some commands (svn log --xml ...), output may be very large:

• Truncate the maximum length of exception messages.
• Summarize outputs in CommandExceptions.

Test Plan: Added execx('cat /usr/share/dict/words && exit 1') to a controller, loaded page. Saw a reasonbly-sized exception onscreen and in the logs.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2193

Differential Revision: https://secure.phabricator.com/D4280