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.
@nmittler - Does it make sense to use HTTP/2 terminology here? I see the [terminated](https://tools.ietf.org/html/draft-ietf-httpbis-http2-14#section-6.4) verbiage in the RST_STREAM summary but would `isRst()` or `isReset()` work?