Raising errors for failed asset generation for webwork

[Oscar Levin]

The cli currently generates webwork passing `False` for the abort_early parameter.  This makes sense so that an author can see as many webwork problems as possible (and get messages for all the failing ones).  But then there is no way for the CLI to know that it should retry building webwork next time.

Could we keep track of failed webwork problems and raise an error at the end of the webwork_to_xml function if there are any failed problems?