Summary:
When you run one copy of arcanist against another copy, you previously got a
meaningless error. After D311 you get a slightly more meaningful error.
Capture the new exception and raise an extremely specific error.
NOTE: Adding arcanist to .arcconfig forces the library conflict error to
trigger; otherwise there's just an implicit conflict because 'arc' reads the
running-copy library amp instead of the working-copy library map. This only
worked before because arcanist includes itself.
Test Plan:
Ran arcanist on a different copy of arcanist, got a good error message.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 312
