Improve UI Error on Unknown Flags in Set Flags

[krkeegan]

Exceptions only show up in the LOG. UI, Error, and Warning are sent out to the user UI. I will add some suggestions on logging to the developer instructions.

There was no need to raise an exception here, plus without a LOG message, the user doesn't get very helpful instructions.

Previous:

Commanding dimmer device 12.29.84 (lamp2) cmd=set_flags                                                                 
ERROR: Error running command set_flags on device 12.29.84 (lamp2) 

Now:

Commanding dimmer device 12.29.84 (lamp2) cmd=set_flags                                                                 
ERROR: Unknown Dimmer flags input: {'on-level'}.                                                                         
Valid flags are: {'ramp_rate', 'on_level', 'backlight'}                                                                
Dimmer set_flags complete

This also highlights the need to abstract more of the code. Rather than have to go around to multiple places to fix this, it should only be in one place.