Fix numerous bugs from our test runs.

Fix numerous bugs from our test runs.

- Only create subdirectories if we're forcing.
- Add support for --limit on mass-migrate
- Match for AlreadyMigratedError as well as fully qualified name.
- Add --force on mass-migrate
- Improve logging messages to remove unnecessary cruft
  and add important information
- Move .git to .git.bak, for record-keeping purposes
- Be more selective about logging for shell
- Resolve error case for exception parsing
- Make exceptions show up by removing --quiet

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>