MINOR: mux-h1: Separate parsing and formatting errors at H1 stream level

MINOR: mux-h1: Separate parsing and formatting errors at H1 stream level

Instead of using H1S flags to report an error on the request or the
response, independently it is a parsing or a formatting error, we now use a
flag to report parsing errors and another one to report formatting
ones. This simplify the message parsing. It is also easier to figure out
what error happened when one of this flag is set. The side may be deduced
checking the H1C_F_IS_BACK flag.