Malformed JSON file no longer raises an unhandled exception.
Malformed JSON file no longer raises an unhandled exception.

Steps to reproduce the issue:
  Refer to https://github.com/cookiecutter/cookiecutter/issues/809
  1. Create a subtly malformed .json file in a project directory
  2. Run cookiecutter on that project
  3. Observe your eyes glazing over as a huge python exception comes out
     of it.

Cause:
  In generate.py's generate_context() function, for a JSON decoding error,
  it raised a friendlier exception.
  That is still daunting to new users.

Solution:
  cli.py's main() function already catches several exceptions.
  I added ContextDecodingException to that list.

  That makes cookiecutter print the friendlier error message
  and exit with a non-zero status.

pjbull/cookiecutter,audreyr/cookiecutter,pjbull/cookiecutter,audreyr/cookiecutter