Better UI for trying to enter a room you're banned from

### Your use case

#### What would you like to do?
When I try to join a call for a room that I am banned from, I would like to see a clearer message that I am banned. Currently, a verbose error is shown.


#### Why would you like to do it?
The current error message makes it seem like something broke. Having a clearer message will better communicate the intentional behaviour of being unable to access a call you are banned from.

#### How would you like to achieve it?
With a custom error message, along the lines of this mockup:


### Have you considered any alternatives?

No, because IMO a custom error message is the clearest solution, is easily achievable, and is easy to replace the existing error message with.

### Additional context

There is already a custom error message for being removed from a room. Something similar should work for a ban message.

Start by locating the code that handles attempts to join a room and the existing custom error message for being removed from a room. Compare the current banned-room error with the linked mockup and make the failure clearly state that access is denied because the user is banned; verify the resulting UI manually.

Written by the indexing model from the issue text.