MINOR: mux-h2/trace: add traces at level ERROR for protocol errors

A number of traces could be added, and a few TRACE_PROTO were replaced
with TRACE_ERROR. The goal is to be able to enable error tracing only
to detect anomalies.

It looks like they're mostly correct as they don't seem to strike on
valid H2 traffic but are very verbose on h2spec.
1 file changed