MINOR: proto_reverse_connect: handle early error before reversal

MINOR: proto_reverse_connect: handle early error before reversal

An error can occured on a reverse connection before accept is completed.
In this case, no parent session can be notified. Instead, wake up the
receiver task on conn_create_mux().

As a counterpart to this, receiver task is extended to match CO_FL_ERROR
flag on pending connection. In this case, the onnection is freed. The
task is then requeued with a 1 second delay to start a new reverse
connection attempt.