Fix error message for forbidden commit titles

Fixes #10

Here's an example of how it looks now:

The poor formatting is due to the line

                    pending_merge_request.comment(f"I couldn't merge this branch: {ex}")

in bot.py where ex is stringified as a tuple of (reason, details). Instead, reason and details should be concatenated, as done elsewhere in the code.

This MR should make the behavior consistent.