Do not post a comment for unrelated errors

[Kobzol]

After every comment on a PR, the bot tries to load the PR information from GitHub. However, this can fail, and in that case the bot starts spamming the PR with error messages. The bot should post a comment when it encounters an error during its operation, but not when it just deals with a comment that is completely unrelated to it. We should first find out whether the comment actually contains a bors command, and if not, we should just always ignore it.

I originally included the PR info loading logic unconditionally to reduce code duplication (so that we always load the PR regardless of the event), but it was a bad idea. We are also needlessly spamming GitHub now, even though most comments are unrelated to bors.