BUG/MINOR: mux-h2: count a protocol error when failing to parse a trailer

BUG/MINOR: mux-h2: count a protocol error when failing to parse a trailer

Commit aab1a60977 ("BUG/MEDIUM: h2/htx: always fail on too large trailers")
explicitly returned an RST_STREAM on failure to decode some trailers, and
used the code H2_ERR_INTERNAL_ERROR. However there are multiple possible
causes for this failure to happen, and it turns out that it's much more
likely to be related to a protocol error than a decompression error. So
let's change this to PROTOCOL_ERROR, and count a protocol error on the
proxy and in the session.

This can be backported to all stable versions (with adjustments related
to these versions, maybe focusing on 3.2 max is reasonable).