BUG/MINOR: mux-h2/stats: not all GOAWAY frames are errors

BUG/MINOR: mux-h2/stats: not all GOAWAY frames are errors

The stats on haproxy.org reported ~12k GOAWAY for ~34k connections, with
only 2 protocol errorss. It turns out that the GOAWAY frame counter added
in commit a8879238c ("MINOR: mux-h2: report detected error on stats")
matches a bit too many situations. First it counts those which are not
sent as well as failed retries, second it counts as errors the cases of
attempts to cleanly close, while it's titled "GOAWAY sent on detected
error". Let's address this by moving the counter up one line and excluding
the clean codes.

This can be backported to 2.3.