Gracefully fail when pandoc is missing.

Instead of:

    Traceback (most recent call last):
      File "setup.py", line 22, in <module>
        long_description = pypandoc.convert(get_path('README.md'), 'rst')
      File "/usr/local/lib/python2.7/dist-packages/pypandoc.py", line 19, in convert
        return _convert(_read_file, _process_file, source, to, format, extra_args)
      File "/usr/local/lib/python2.7/dist-packages/pypandoc.py", line 37, in _convert
        from_formats, to_formats = get_pandoc_formats()
      File "/usr/local/lib/python2.7/dist-packages/pypandoc.py", line 74, in get_pandoc_formats
        stdout=subprocess.PIPE)
      File "/usr/lib/python2.7/subprocess.py", line 709, in __init__
        errread, errwrite)
      File "/usr/lib/python2.7/subprocess.py", line 1326, in _execute_child
        raise child_exception
    OSError: [Errno 2] No such file or directory