Registering with a used username freezes the screen without feedback

When registering with an already registered username the screen freezes without showing an error. By looking at the responses I see the server returns a status 500 {"errors":{"detail":"Internal server error"}}

I assume this specific case will be solved when the server returns something similar to when an already used mail address is used, which is a status 400 {"error":"{\"email\":[\"has already been taken\"]}"} , so I already made pleroma#646 (closed)

However, when this error happens (which may or may not happen in the future because of other reasons)

1. the screen shouldn't freeze and
2. an error should be displayed so that the end user at least knows something went wrong