nbdistribute --template fail gracefully

[betatim]

Have nbdistribute --template fail gracefully when the repo already exists.

[lwasser]

Currently when the repo exists, you get this message:

(earth-analytics-python) MacBook-Pro-4:autograded-course-starter leah-su$ nbdistribute --template
Using /Users/leah-su/Documents/github/2-autograding/autograded-course-starter/student to create the student template.
Loading configuration from config.yml
Creating template repository.
Traceback (most recent call last):
  File "/Users/leah-su/anaconda3/envs/earth-analytics-python/bin/nbdistribute", line 11, in <module>
    sys.exit(distribute())
  File "/Users/leah-su/anaconda3/envs/earth-analytics-python/lib/python3.6/site-packages/grading/__main__.py", line 131, in distribute
    config['github']['token'])
  File "/Users/leah-su/anaconda3/envs/earth-analytics-python/lib/python3.6/site-packages/grading/github.py", line 55, in create_repo
    organization.create_repository(repository, title)
  File "/Users/leah-su/anaconda3/envs/earth-analytics-python/lib/python3.6/site-packages/github3/decorators.py", line 30, in auth_wrapper
    return func(self, *args, **kwargs)
  File "/Users/leah-su/anaconda3/envs/earth-analytics-python/lib/python3.6/site-packages/github3/orgs.py", line 630, in create_repository
    json = self._json(self._post(url, data), 201)
  File "/Users/leah-su/anaconda3/envs/earth-analytics-python/lib/python3.6/site-packages/github3/models.py", line 156, in _json
    raise exceptions.error_for(response)
github3.exceptions.UnprocessableEntity: 422 Repository creation failed.

it would be nice for it to leave a nice little message like
"i already did that dumb@ss" :) or something even nicer?!