Opaque exception when shape args are missing

[scal444]

bumpy.py -f examples/input_pdbs/small_flat_bilayer.pdb -s sphere -z 10

yields:
File "./bumpy.py", line 1498, in
main()
File "./bumpy.py", line 1418, in main
check_argument_sanity(args)
File "./bumpy.py", line 1332, in check_argument_sanity
geometric_args = {garg.split(':')[0] : float(garg.split(':')[1]) for garg in args.g }
TypeError: 'NoneType' object is not iterable

We should catch this sort of thing and print out a human readable error