Provide a more useful error for invalid use case.

The previous help statement was not very usefull for users who
don't know how to use nbconvert (ironically, the exact set of
users who would encounter the error).

Output for `ipython nbconvert` failure now reads:

    Examples
    --------

        The simplest way to use nbconvert is
        
        > ipython nbconvert mynotebook.ipynb
        
        which will convert mynotebook.ipynb to the default format (probably HTML).
        
        You can specify the export format with `--format`.
        Options include ['basic_html', 'full_html', 'latex', 'markdown', 'python', 'reveal', 'rst', 'sphinx_howto', 'sphinx_manual']
        
        > ipython nbconvert --format latex mynotebook.ipnynb
        
        You can also pipe the output to stdout, rather than a file
        
        > ipython nbconvert mynotebook.ipynb --stdout
        
        Multiple notebooks can be given at the command line in a couple of 
        different ways:
        
        > ipython nbconvert notebook*.ipynb
        > ipython nbconvert notebook1.ipynb notebook2.ipynb
        
        or you can specify the notebooks list in a config file, containing::
        
            c.NbConvertApp.notebooks = ["my_notebook.ipynb"]
        
        > ipython nbconvert --config mycfg.py

    For help, use "ipython nbconvert --help"