Bluetooth: CAP: Add support for handling ASE errors

If we get an error/rejection from the CAP acceptor when
performing the Unicast Audio Start or Stop procedure then
we need to abort the procedure and let the application determine
what the next step is.

This change triggered a corner case when connecting to multiple
CAP acceptors as the CAP initiatior. This was also fixed as part
of this.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>