Error checks on simulated direct VFO addressing

Many APIs  emulate direct VFO  addressing by using  caps->set_vfo() or
caps->vfo_op(..., RIG_OP_TOGGLE)  to simulate  direct addressing  of a
non-current  VFO. The  mirrored function  call to  revert back  to the
original  current  VFO was  not  checking  for  errors.  This  is  not
unreasonable but it turns out that some rigs can arbitarily fail doing
VFO changes or swaps (e.g. Yaesu FT-450).

I have added checks to these calls but only fail with the error if the
API call being wrapped does not fail itself. This allows the mirroring
VFO  operation to  be  completed,  if possible,  even  if the  wrapped
function could not be completed,  hence as before, making every effort
not to change the rig state unnecessarily.