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 http://github.com/bhollis/rbtools/commits/errorhandling/. 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.