Improvements to post-review error-handling with Perforce

Review Request #1886 — Created Nov. 1, 2010 and submitted — Latest diff uploaded




A handful of changes to improve error handling in post-review.

* Die with an error when the provided Perforce changelist does not exist
* Die with an error if diffs are empty
* Die with an error when there are no changes in a Perforce changelist. Fixes issue 832.
* Default to the default changelist when no Perforce changelist is provided. Fixes issue 1191.

This corresponds to the top four commits at Note that 2bd8897b133aa81e2b32 changes the default of running "post-review" in a Perforce client to uploading the default change - the other changes should not introduce any new behavior.
These changes have been in use for a while.