Changing stream verification to throw Http2StreamException.

Motivation:

Currently when receiving DATA/HEADERS frames, we throw Http2Exception (a
connection error) instead of Http2StreamException (stream error).  This
is incorrect according to the HTTP/2 spec.

Modifications:

Change the Http2Stream.verifyState method to throw Http2StreamException.

Result:

Stream state verification is properly handled.
I wouldn't be heart broken if the immediate removal policy went away in the general case...but it may be useful for simpler clients or more controlled use cases.  What if we just change the default policy?