Inappropriate "invalid argument error" in "update the event map" section when trying to remove a not yet subscribed event

[whimboo]

Step 6.2.1.1 of update the event map instructs to raise an invalid argument error. At that point the eventName has already been proven to be valid. As such the error type is inappropriate here.

Instead we should raise an error like event not subscribed.

We don't have an errors section specific for WebDriver BiDi yet, so it might make sense to get started now.