Add user-friendly error message to `generatekeys.py`

[aaaaalbert]

While generatekeys.py isn't the most-often used script in our repos, it should nevertheless print meaningful information when called with wrong or too little arguments. Let usage() functions in other code inspire you.

albert$ python generatekeys.py 
Traceback (most recent call last):
  File "generatekeys.py", line 14, in <module>
    pubfn = sys.argv[1]+'.publickey'
IndexError: list index out of range

albert$ python generatekeys.py keyname abcdef
Traceback (most recent call last):
  File "generatekeys.py", line 18, in <module>
    keylength = int(sys.argv[2])
ValueError: invalid literal for int() with base 10: 'abcdef'