Warn instead of aborting on invalid values for append_revisions_only.

* bzrlib/tests/test_branch.py:
(TestBranchOptions.test_invalid_append_revisions_only): Cath the
warning instead of the exception.

* bzrlib/branch.py:
(BzrBranch8._get_append_revisions_only): Only warn if the option
is invalid and default to False in that case for backward
compatibility.