Provide custom messages for common errors

[varkor]

Some errors are extremely common in Rust pull requests, for example: not passing the tidy check. In these cases, the workflow could be simplified even more, by displaying a comment at the top of the bot's message (accompanying the usual output), for example:

Run ./x.py test tidy to identify rustc style errors.
Your code fails the tidy check on the following lines:
[...]

This may be going over the top, reducing the single click to open the spoiler box; feel free to close if it's not worth the effort! It does let reviewers more quickly scan to see what the problem is, though.