M_UNKNOWN_TOKEN should be reported in a friendly way.

Currently, if a dodgy access token is given, then this mouthful is spat at the user:

`Error occurred during test: VoIPTesterError: M_UNKNOWN_TOKEN Invalid macaroon passed. undefined [Can't get TURN credentials from homeserver]`

It would be nice to nicen this up — `M_UNKNOWN_TOKEN` seems like it should be the right thing to match against; I'm confident the spec will agree with that.