MINOR: proto_reverse_connect: refactor preconnect failure

MINOR: proto_reverse_connect: refactor preconnect failure

When a connection is freed during preconnect before reversal, the error
must be notified to the listener to remove any connection reference and
rearm a new preconnect attempt. Currently, this can occur through 2 code
paths :
* conn_free() called directly by H2 mux
* error during conn_create_mux(). For this case, connection is flagged
  with CO_FL_ERROR and reverse_connect task is woken up. The process
  task handler is then responsible to call conn_free() for such
  connection.

Duplicated steps where done both in conn_free() and process task
handler. These are now removed. To facilitate code maintenance,
dedicated operation have been centralized in a new function
rev_notify_preconn_err() which is called by conn_free().