Sending cheques return value

[Eknir]

In swap/swap.go sendCheque, we send a cheque to a peer, but never check whether sending the cheque succeeds.
The function sendCheque should take into account the possibility of sending not succeeding, attempt to re-send (if the error condition allows this) and not reset the balance if sending is not possible.

[mortelli]

Should be completed along with/after #1601 and #1635.

[ralph-pichler]

The error is now handled (albeit not optimally) and the pending cheque mechanism takes care of the resending.