MINOR: stconn: Add a flag to ack endpoint errors at SC level

MINOR: stconn: Add a flag to ack endpoint errors at SC level

The flag SC_FL_ERROR is added to ack errors on the endpoint. When
SE_FL_ERROR flag is detected on the SE descriptor, the corresponding is set
on the SC. Idea is to avoid, as far as possible, to manipulated the SE
descriptor in upper layers and know when an error in the endpoint is handled
by the SC.

For now, this flag is only set and cleared but never tested.