APIs that commit should make a distinction between "The commit succeeded" and
"an error occurred", since it is possible for both to happen.
svn_fs_commit_txn() makes this distinction. svn_repos_get_commit_editor5() does
too, albeit in an indirect way (via the COMMIT_CALLBACK parameter).
svn_editor_complete() does not.
svn_editor_complete() should grow a way to express the multitude of success
notions that we have:
1. Commit succeeded (new revision was created)
2. Post-commit FS processing (insert into rep-cache.db) failed
3. Post-commit hook failed
4. Post-commit repos processing (invocation of commit_callback) failed
Major