Better handle of invalid UPS name

[gbakeman]

WinNUT currently does not do a good job of communicating to the user when they specify an invalid UPS name is their configuration, nor does it take advantage of the LIST UPS command available in the NUT protocol which could further assist the user in selecting their UPS. Some changes that could be made:

• Present an error of some kind to the user when an invalid UPS name is specified
• Create a dialog that can be shown after connecting to a NUT server that displays a list of available UPS devices
  • Account for the fact that some NUT server distributions do not support the LIST UPS command, and allow the user to manually input the UPS name