vignette_from_args fails

[dimalvovs]

to reproduce:

(.env) u88@clarity-3 pycogaps % python vignette_from_args.py 

______      _____       _____   ___  ______  _____ 
| ___ \    /  __ \     |  __ \ / _ \ | ___ \/  ___|
| |_/ /   _| /  \/ ___ | |  \// /_\ \| |_/ /\ `--. 
|  __/ | | | |    / _ \| | __ |  _  ||  __/  `--. |
| |  | |_| | \__/\ (_) | |_\ \| | | || |    /\__/ /
\_|   \__, |\____/\___/ \____/\_| |_/\_|    \____/ 
       __/ |                                       
      |___/             
                                 
                    
pycogaps version  0.0.1
This vignette was built using pycogaps version 0.0.1
Traceback (most recent call last):
  File "/Users/u88/Documents/jhu/cogaps/pycogaps/vignette_from_args.py", line 16, in <module>
    params_file = sys.argv[1]
IndexError: list index out of range

and also if the params.yaml file is provided as input:

(.env) u88@clarity-3 pycogaps % python vignette_from_args.py params.yaml 

______      _____       _____   ___  ______  _____ 
| ___ \    /  __ \     |  __ \ / _ \ | ___ \/  ___|
| |_/ /   _| /  \/ ___ | |  \// /_\ \| |_/ /\ `--. 
|  __/ | | | |    / _ \| | __ |  _  ||  __/  `--. |
| |  | |_| | \__/\ (_) | |_\ \| | | || |    /\__/ /
\_|   \__, |\____/\___/ \____/\_| |_/\_|    \____/ 
       __/ |                                       
      |___/             
                                 
                    
pycogaps version  0.0.1
This vignette was built using pycogaps version 0.0.1
Traceback (most recent call last):
  File "/Users/u88/Documents/jhu/cogaps/pycogaps/vignette_from_args.py", line 20, in <module>
    os.mkdir(outdir)
OSError: [Errno 30] Read-only file system: '/output/'

Lets:

1. fail more gracefully (invite user to specify params.yaml) if its missing;
2. fix the reason for failure even if params file is provided.