Start addressing Andrew's concerns.

* bzrlib/tests/test_conflicts.py:
(TestParametrizedResolveConflicts): Add a docstring.
(TestResolvePathConflictBefore531967.assertPathConflict): Really
inject the old fashioned conflict object.

* bzrlib/merge.py:
(Merge3Merger._merge_names): Better explain why we shouldn't
generate helpers at this point.
(Merge3Merger.cook_conflicts): Be more verbose when failing.

* bzrlib/conflicts.py:
(PathConflict._resolve): Be more verbose when failing.