current combinatorics of the cmdline specifications lead to suboptimal reporting of incorrect specification

[yarikoptic]

in all of those cases I expect us to report usage synopsis with some informative message instead of code tracebacks:

examples:

• [x] no options specified whatsoever:

  $> heudiconv
  INFO: Running heudiconv version 0.4
  Traceback (most recent call last):
  File "/usr/bin/heudiconv", line 11, in <module>
  load_entry_point('heudiconv==0.4', 'console_scripts', 'heudiconv')()
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 120, in main
  process_args(args)
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 240, in process_args
  heuristic = load_heuristic(args.heuristic)
  File "/usr/lib/python2.7/dist-packages/heudiconv/utils.py", line 274, in load_heuristic
  if os.path.sep in heuristic or os.path.lexists(heuristic):
  TypeError: argument of type 'NoneType' is not iterable

• [x] heuristic specified by no subject template or files:

  $> heudiconv -f reproin
  INFO: Running heudiconv version 0.4
  Traceback (most recent call last):
  File "/usr/bin/heudiconv", line 11, in <module>
  load_entry_point('heudiconv==0.4', 'console_scripts', 'heudiconv')()
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 120, in main
  process_args(args)
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 244, in process_args
  args.subjs, grouping=args.grouping)
  File "/usr/lib/python2.7/dist-packages/heudiconv/parser.py", line 155, in get_study_sessions
  for f in files_opt:
  TypeError: 'NoneType' object is not iterable

• [ ] ls command should work also without heuristic specified:

  $> heudiconv  --command ls --files ~/picts/mris/dartmouth-phantoms/bids_test4-20161014/phantom-1/anat-angio_acq-fl3d                                                                                                         
  Traceback (most recent call last):
  File "/usr/bin/heudiconv", line 11, in <module>
  load_entry_point('heudiconv==0.4', 'console_scripts', 'heudiconv')()
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 120, in main
  process_args(args)
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 230, in process_args
  process_extra_commands(outdir, args)
  File "/usr/lib/python2.7/dist-packages/heudiconv/cli/run.py", line 63, in process_extra_commands
  heuristic = load_heuristic(args.heuristic)
  File "/usr/lib/python2.7/dist-packages/heudiconv/utils.py", line 274, in load_heuristic
  if os.path.sep in heuristic or os.path.lexists(heuristic):
  TypeError: argument of type 'NoneType' is not iterable

[mgxd]

partially addressed in #209