Wrong error type in error callback

[roddolf]

The typings of the errorCallback have that the returned error is a string, but in reality its an error Frame or a CloseEvent.

[JSteunou]

I assume you are talking about the typescript definition. I am not maintaining it I accept PR from typescript community. If the typescript community is not capable of maintaining it I will remove it. That was my only condition when I accepted the PR that add it the 1st time.