BUG/MINOR: mux-quic: fix transport VS app CONNECTION_CLOSE

BUG/MINOR: mux-quic: fix transport VS app CONNECTION_CLOSE

A recent series of patch were introduced to streamline error generation
by QUIC MUX. However, a regression was introduced : every error
generated by the MUX was built as CONNECTION_CLOSE_APP frame, whereas it
should be only for H3/QPACK errors.

Fix this by adding an argument <app> in qcc_set_error. When false, a
standard CONNECTION_CLOSE is used as error.

This bug was detected by QUIC tracker with the following tests
"stop_sending" and "server_flow_control" which requires a
CONNECTION_CLOSE frame.

This must be backported up to 2.7.