Show a better message if fails to detect plugin

$ rows convert http://bit.ly/todas-olimpiadas all-games.sqlite
Traceback (most recent call last):
  File "/home/turicas/projects/rows/rows/utils.py", line 266, in import_from_source
    import_function = getattr(rows, 'import_from_{}'.format(plugin_name))
AttributeError: module 'rows' has no attribute 'import_from_None'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/turicas/.virtualenvs/rows/bin/rows", line 11, in <module>
    load_entry_point('rows', 'console_scripts', 'rows')()
  File "/home/turicas/.virtualenvs/rows/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/turicas/.virtualenvs/rows/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/turicas/.virtualenvs/rows/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/turicas/.virtualenvs/rows/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/turicas/.virtualenvs/rows/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/turicas/projects/rows/rows/cli.py", line 104, in convert
    verify_ssl=verify_ssl)
  File "/home/turicas/projects/rows/rows/cli.py", line 52, in _import_table
    encoding=encoding, *args, **kwargs)
  File "/home/turicas/projects/rows/rows/utils.py", line 285, in import_from_uri
    return import_from_source(source, default_encoding, *args, **kwargs)
  File "/home/turicas/projects/rows/rows/utils.py", line 268, in import_from_source
    raise ValueError('Plugin (import) "{}" not found'.format(plugin_name))
ValueError: Plugin (import) "None" not found