make generator_cmd in bloom-generate required

[pseyfert]

This handles when bloom-generate gets called without argument and
aborts processing.
This fixes #580.

[pseyfert]

FTR: my original commit added add_subparsers(..., required=True). That kwarg appears to be available starting in python 3.7 https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser.add_subparsers
i.e. on ubuntu 18.04 LTS (where python3 is at 3.6.9) that fix would have broken bloom generate.

following https://bugs.python.org/issue9253#msg186387
python 3.6 needs this fix as

subparser = add_subparsers(...)
subparser.required = True # compatible with python 3.6

instead of

subparser = add_subparsers(... required=True) # incompatible with python3.6 and 2.7

In python 2.7 add_subparsers already behaves by default like the python 3 version with required=True. (similar to python 3.6, using a kwarg would break processing on python 2.7. setting the required attribute of subparser doesn't break)