There was a design problem earlier where the serve goroutine assumed
that the readFrame goroutine could return only connection-level
errors, but the readFrames goroutine (and the underlying Framer)
assumed it could return stream-level errors (type StreamError) and
have them handled as stream errors in the layers above. That's how it
should have been, and what this CL does.

Now readFrames returns both the Frame and error from ReadFrames
together as a pair, and an error isn't necessarily fatal to the
connection.

Fixes golang/go#12733
Fixes bradfitz/http2#53

Change-Id: If4406ceaa019886893d3c61e6bfce25ef74560d3
Reviewed-on: https://go-review.googlesource.com/15735Reviewed-by: Dmitry Vyukov <dvyukov@google.com>