abdt_exception: doc: describe 'base_name' properly
abdt_branch: style: 2 lines after last fn
abdcmnt_commenter.py: style: use str.format()

Make it easier and more robust to compose the error message by using
format instead of string concatenation.

Test Plan:
$ ./testbed/arcyd/acceptance_tests.sh
inspect 'test_merge_conflict' review
Arcyd: comment on review if fail push on landing

When landing a review, if the merge succeeds but the push fails then
update the review with the exception and move the review to
'needs revision'.

This gives the reviewer the option of troubleshooting the failed push
and electing to retry by accepting the review again.

Test Plan:
Added coverage in
$ ./testbed/arcyd/acceptance_tests.sh
and verified result