Add discord.Guild converter and GuildNotFound error

[AbstractUmbra]

Summary

This adds a discord.Guild converter as well as the resulting error if a match is not found.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

Historically I'm against such a converter but I guess if the shortcomings are documented it's okay, even though I think the use cases for it aren't that big.

[Rapptz]

This is missing the documentation changes required in api.rst

Mainly, adding the class to the list of converters and adding the exception to the exception hierarchy and the list of exceptions.