Please tell me when I give Mjolnir invalid MXID

[Mikaela]

Is your feature request related to a problem? Please describe.

Whenever I do something wrong, Mjolnir tells me "There was an error processing your command - see console/log for details" (#162) which feels a bit frustating as I don't hace access to the log and the issue may be something very simple.

Describe the solution you'd like

When I give Mjolnir an email address or XMPPID while specifying I am banning/unbanning an user, I would like Mjolnir to tell me that Matrix IDs are e.g. @poke:example.net not [email protected] or even @[email protected].

Describe alternatives you've considered

#162 might be a viable alternative, but likely give a confusing error message that may not open so easily as a user error.

Additional context

I got three phone screenfuls of silliness and pinged the one running the instance before I realized that I am trying to speak XMPP or email at it 😅

[turt2live]

Effective duplicate of #64