Expose the Invalid Room Alias error in a better way

[akirk]

When you set an invalid alias, you can only see this in the developer tools of the browser

HomeServerError: Room alias invalid on GET https://wporg.automattrix.com/_matrix/client/v3/directory/room/%20%23training%3Acommunity.wordpress.org?

• [ ] Validate the room alias field for syntax (in the above example I had an accidental space in front of the room name).
• [ ] Expose the error message if the room alias is invalid despite being syntactically correct.