http2: aways emit goaway error and teardown the connection independent of the error code

a case was reported where a goaway error was received with a cancel error code; this code seems to have been misused by the peer server, since it is a stream error code, however the client should react to it by just closing it anyway, as the http2 parser already moved on to half-closed anyway, so not accepting new streams anymore

fixes github.com/HoneyryderChuck/httpx/issues/102