Don't log exception traceback on Forbidden for welcomes.

[scragly]

Closes #763.

Description

Members have a welcome DM sent to them on accepting rules, which may raise a discord.Forbidden exception in the case they have DMs disabled in their privacy settings.

The current implementation catches a broad Exception and uses log.exception to allow the error to be logged fully with traceback as an exception without interrupting the code.

This is an unnecessary log since it's something by design, not an error, and the broad Exception catching is against the more explicit exception catching we prefer.

Changes

The except block only catches discord.Forbidden for the single known case we're intending to handle and suppress.

To ensure the original accept command message is still removed, it's placed in a finally block. This allows unknown errors to raise correctly and propagate to the appropriate error logging and handling.

[SebastiaanZ]

Looks good.