HTTP/3: adjusted QUIC connection finalization.

HTTP/3: adjusted QUIC connection finalization.

When an HTTP/3 function returns an error in context of a QUIC stream, it's
this function's responsibility now to finalize the entire QUIC connection
with the right code, if required.  Previously, QUIC connection finalization
could be done both outside and inside such functions.  The new rule follows
a similar rule for logging, leads to cleaner code, and allows to provide more
details about the error.

While here, a few error cases are no longer treated as fatal and QUIC connection
is no longer finalized in these cases.  A few other cases now lead to
stream reset instead of connection finalization.