Created by: dpursehouse
Summary:
(1.) When the .buckversion file specifies a revision that is not present in the current repository, print a message stating that Buck is fetching updates.
(2.) When the git fetch fails, only display a "failed" message. Move the bit about how the user can disable the update into the informational message that was already displayed as described in (1.).
(3.) If the revision specified in .buckversion does not exist on the remote, either because it's plain wrong or because the revision is on a remote that is not configured in the local repository, the git checkout fails. Catch the exception and display a message rather than a Python Traceback.
Test Plan:
Set .buckversion to a revision that does not exist. Run buck clean and
observe that it prints the message in (1.), then runs git fetch, and then
fails to check out the revision and fails with an error message.