Improvements for Substrate #8409's bugs

[joao-paulo-parity]

Description

Targets the problems raised in #256

• Ignore comments from bots
  • Context: this prevents what happened in update README.adoc substrate#8409 since the Bot should no longer be able to react to its own events
• Improve error handling
  • Context: before, error path was always posting a new Github comment indiscriminately. Now we only attempt to post a message if the comment event has not be initiated by the bot.
• Have safer APIs based on Option and comment reasoning for being cautious around them
  • Context: part of the bug in update README.adoc substrate#8409 was due to the code expecting a field which might be missing in the API; preventive field checks motivated by safer APIs will prevent those code paths from being reached in the first place.

closes #256