MINOR: h3: adjust error reporting on receive

MINOR: h3: adjust error reporting on receive

This commit is the second step to simplify HTTP/3 error management. This
times it deals with receive side on h3_rcv_buf().

Various internal HTTP/3 to HTX conversion functions does not set
H3_INTERNAL_ERROR on h3c err anymore. Only standard error code are set.
For every errors, both internal and protocol ones, a negative value is
returned. This ensure that h3_rcv_buf() looping is interrupted. This
function will then set H3_INTERNAL_ERROR only if no standard error is
registered via h3c or h3s.

Along the previous commit, this should better reflect internal errors
from protocol ones caused by a faulty client.