Add support for showing generic errors
Mystic supports flashing feild-specific errors like "please include a description" or "that username is already taken". However, some errors are not associated with a specific field, and are reported as generic errors. These errors are mostly internal, but still crop up from time to time, especially during development. However, there is currently no UI for showing these, so they end up silently ignored.
Completion Criteria:
- When a generic error is flashed, the UI presents it
- This feature is available for all pages