BUG/MEDIUM: mux-h2: Don't report error on SE if error is only pending on H2C

BUG/MEDIUM: mux-h2: Don't report error on SE if error is only pending on H2C

In h2s_wake_one_stream(), we must not report an error on the stream-endpoint
descriptor if the error is not definitive on the H2 connection. A pending
error on the H2 connection means there are potentially remaining data to be
demux. It is important to not truncate a message for a stream.

This patch is part of a series that should fix a bug reported in issue #2388
(#2388#issuecomment-1855735144). Backport instructions will be shipped in
the last commit of the series.