MINOR: h3: use INTERNAL_ERROR code for init failure

MINOR: h3: use INTERNAL_ERROR code for init failure

Consider that application layer is responsible to set proper error code
on init or finalize operation failure. In case of H3, use INTERNAL_ERROR
application error code. This allows to remove qcc_set_error() invocation
from qmux_init().

In case application layer would not specify any error code, fallback
INTERNAL_ERROR transport error code would be used thanks to the recent
change introduced for error management in qmux_init().