BUG/MINOR: h3: fix error code on incomplete request

BUG/MINOR: h3: fix error code on incomplete request

HTTP/3 parser validates that a request has been received when FIN is
signalled by the peer on a request stream. FIN signal may be conveyed
with an extension frame. In this case, if no header was received, the
request is invalid and the stream is closed with an error.

This patch simply fixes the reported error code to H3_REQUEST_INCOMPLETE
as recommended by RFC 9114 when the issue occurs on the frontend side.
H3_MESSAGE_ERROR is kept on the backend side as there is no specific
error code defined for an incomplete response.

This must be backported up to 2.8. Ensure enum is adapted for 2.8 as the
name changed after.