MINOR: h3: check connection error during sending

MINOR: h3: check connection error during sending

If an error occurs during HTX to H3 encoding, h3_snd_buf() should be
interrupted. This commit add this possibility by checking for <h3c.err>
member value. If non null, sending loop is stopped and an error is
reported using qcc_set_error().

This commit does not change any behavior for now, as <h3c.err> is never
set during sending. However, this will change in future commits, most
notably to reject too many headers or handle buffer allocation failure.
As such, this commit should be backported along the following fixes.
Note that in 2.6 qcc_set_error() does not exist and must be replaced by
qcc_emit_cc_app().