extend led() function to accept colour names & proper exit

hey... because nobody can remember the numbers 1-18 or wants to count them everytime, the led() function now accepts strings like "red1" which means the red led on arm1 or "blue3" which means the blue led on the third arm and so on... so no knowledge about the numbering scheme of the leds is needed (just about which is which arm)... incl documentation...

and i added "exits" after errors to avoid tracebacks...

ben