BUG/MINOR: mux-h1: Use configured error files if possible for early H1 errors

BUG/MINOR: mux-h1: Use configured error files if possible for early H1 errors

The H1 multiplexer is able to produce some errors on its own to report early
errors, before the stream is created. In that case, the error files of the
proxy were tested to detect empty files (or /dev/null) but they were not
used to produce the error itself.

But the documentation states that configured error files are used in all
cases. And in fact, it is not really a problem to use these files. We must
just format a full HTX message. Thanks to the previous patch, it is now
possible.

This patch should fix the issue #3032. It should be backported to 3.2. For
older versions, it must be discussed but it should be quite easy to do.