BUG/MEDIUM: h2/htx: always fail on too large trailers

BUG/MEDIUM: h2/htx: always fail on too large trailers

In case a header frame carrying trailers just fits into the HTX buffer
but leaves no room for the EOM block, we used to return the same code
as the one indicating we're missing data. This could would result in
such frames causing timeouts instead of immediate clean aborts. Now
they are properly reported as stream errors (since the frame was
decoded and the compression context is still synchronized).

This must be backported to 1.9.