Clarifies NotImplementedError in part
Clarifies NotImplementedError in part

Part requires the child class to implement `to_xml`, but the error just
reads

~~~
NotImplementedError:
 NotImplementedError
~~~

This could lead to some confusion so we updated the message to say what
child class and what method needs to be implemented.

Amos King @adkron <amos@binarynoggin.com>
Bob Lail @boblail <bob.lailfamily@gmail.com>

concordia-publishing-house/open_xml_package,openxml/openxml-package