Give an error when a template is not found

[maartenbreddels]

Closes #129
Example:

$ voila notebooks/basics.ipynb  --template=bla   
Traceback (most recent call last):
  File "/Users/maartenbreddels/miniconda3/envs/voila/bin/voila", line 11, in <module>
    load_entry_point('voila', 'console_scripts', 'voila')()
  File "/Users/maartenbreddels/miniconda3/envs/voila/lib/python3.6/site-packages/traitlets/config/application.py", line 657, in launch_instance
    app.initialize(argv)
  File "/Users/maartenbreddels/src/QuantStack/voila/voila/app.py", line 294, in initialize
    self.setup_template_dirs()
  File "/Users/maartenbreddels/src/QuantStack/voila/voila/app.py", line 303, in setup_template_dirs
    self.template)
  File "/Users/maartenbreddels/src/QuantStack/voila/voila/paths.py", line 78, in collect_template_paths
    raise ValueError('No template sub-directory with name %r found in the following paths:\n\t%s' % (template_name, paths))
ValueError: No template sub-directory with name 'bla' found in the following paths:
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template
	/Users/maartenbreddels/Library/Jupyter/voila/template
	/Users/maartenbreddels/miniconda3/envs/voila/share/jupyter/voila/template
	/usr/local/share/jupyter/voila/template
	/usr/share/jupyter/voila/template

[SylvainCorlay]

This looks good to me.