nbconvert: 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:

    This application is used to convert notebook files (*.ipynb) to various other
    formats.

    Options
    -------

    Arguments that take values are actually convenience aliases to full
    Configurables, whose aliases are listed on the help line. For more information
    on full configurables, see '--help-all'.

    --debug
        set log level to logging.DEBUG (maximize logging output)
    --init
        Initialize profile with default config files.  This is equivalent
        to running `ipython profile create <profile>` prior to startup.
    --quiet
        set log level to logging.CRITICAL (minimize logging output)
    --stdout
        Write notebook output to stdout instead of files.
    --profile=<Unicode> (BaseIPythonApplication.profile)
        Default: u'default'
        The IPython profile to use.
    --notebooks=<List> (NbConvertApp.notebooks)
        Default: []
        List of notebooks to convert. Wildcards are supported. Filenames passed
        positionally will be added to the list.
    --ipython-dir=<Unicode> (BaseIPythonApplication.ipython_dir)
        Default: u'/home/jonathan/.config/ipython'
        The name of the IPython directory. This directory is used for logging
        configuration (through profiles), history storage, etc. The default is
        usually $HOME/.ipython. This options can also be specified through the
        environment variable IPYTHONDIR.
    --format=<CaselessStrEnum> (NbConvertApp.export_format)
        Default: 'full_html'
        Choices: ['basic_html', 'full_html', 'latex', 'markdown', 'python', 'reveal', 'rst', 'sphinx_howto', 'sphinx_manual']
        The export format to be used.
    --writer=<DottedObjectName> (NbConvertApp.writer_class)
        Default: 'FilesWriter'
        Writer class used to write the  results of the conversion
    --config=<Unicode> (BaseIPythonApplication.extra_config_file)
        Default: u''
        Path to an extra config file to load.
        If specified, load this config file in addition to any other IPython config.
    --log-level=<Enum> (Application.log_level)
        Default: 30
        Choices: (0, 10, 20, 30, 40, 50, 'DEBUG', 'INFO', 'WARN', 'ERROR', 'CRITICAL')
        Set the log level by value or name.

    To see all available configurables, use `--help-all`

    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