Raise an error on unknown Bitcoin private key

Without this, when an invalid private key is entered, we get this exception:

> Traceback (most recent call last):
  File "convert.py", line 91, in <module>
    print(numtowif(wiftonum(args.key)))
  File "convert.py", line 61, in numtowif
    step1 = chr(args.version)+numpriv
AttributeError: 'Namespace' object has no attribute 'version'

This makes the error more explicit.