Currently, if we are unable to reset a PCI device we return a fairly
generic 'No PCI reset capability available' error message.

Fix that by returning an error from the individual reset messages and
using that error to construct the higher level error mesage.

* src/pci.c: set errors in pciTryPowerManagementReset() and
  pciTrySecondaryBusReset() on failure; use those error messages
  in pciResetDevice(), or explain that no reset support is available