Expose the Invalid Room Alias error in a better way

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.

Start by locating the room alias input and the request handling for the Matrix room-directory lookup. Reproduce the leading-space example, then verify both checklist items: syntactically invalid aliases are rejected in the field, and valid-looking aliases that the server rejects display the error outside browser developer tools.

Written by the indexing model from the issue text.