[Highlight]: 50007 error when users got dms locked.

[Evanroby]

I have recently seen that, highlight tries to dm every users that turned an highlight on.
But then, if user got dms closed, or even blocked the bot, we get a 50007 error in console.

Would you like me to make a PR? the idea would be to wrap the send part with a try/except, and if we raise discord.Forbidden, we send something like "{highlighted_usr.mention} I tried to notify you but your DMs are closed.", or just silently pass.

Would you like me to PR this, or do you not want it?