Revert "BUG/MEDIUM: mux-h2: make sure to always report pending errors to the stream"

Revert "BUG/MEDIUM: mux-h2: make sure to always report pending errors to the stream"

This reverts commit 44932b6c417e472d25039ec3d7b8bf14e07629bc.

The patch above was only necessary to handle partial headers or trailers
parsing. There was nothing to prevent the H2 multiplexer to start to add
headers or trailers in an HTX message and to stop the processing on error,
leaving the HTX message with no EOH/EOT block.

From the HTX API point of view, it is unexepected. And this was fixed thanks
to the commit ba7dc46a9 ("BUG/MINOR: h2/h3: Never insert partial
headers/trailers in an HTX message").

So this patch can be reverted. It is important to not report a parsign error
too early, when there are still data to transfer to the upper layer.

This patch must be backport where 44932b6c4 was backported but only after
backporting ba7dc46a9 first.