Fix pairing failure detection and race condition during pairing

ret_error is only set for local DBus error.
We also need to check the BlueZ DBus error in the message, e.g. for timeout or authentication canceled by the user on the remote device.

The 3268989d commit introduced a race condition for remote initiated pairing, removing the remote device before the pairing is completed.
pairing variable should be set to 1 in request_* functions to prevent this removal, as in interface_added_handler for locally initiated pairing.

Signed-off-by: Frédéric Danis frederic.danis@collabora.com