Not throw a Http2Exception when a ChannelFuture is returned

Motivation:
At the moment we have some methods that return a ChannelFuture but still throw a Http2Exception too. This is confusing in terms of semantic. A method which returns a ChannelFuture should not throw an Http2Exception but just fail the ChannelFuture.

Modifications:
* Make sure we fail the returned ChannelFuture in cases of Http2Exception and remove the throws Http2Exception from the method signature.
* Also some cleanup

Result:
Make the API usage more clear.