Avoidance of `KeyError` from `"draft"` key when pulling GitHub PR data.

[Xithrius]

Closes #928

We want to ensure the information received from GitHub is not malformed.

Due to this, FetchError handling has been moved to fetch_data function, so no placeholder variables in returns need to be used.

FetchError is no longer a data class, but an actual error which inherets from Exception.

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[GDWR]

Next, we need to add a cog error handler so raised FetchErrors are displayed to the users in a neatly formatted embed.

[Xithrius]

@GDWR Would you like to take over this PR?

[Xithrius]

Will finish this at a later time. Closing, since it'll become stale in the meantime.