Avoid throwing errors if MPImportServiceContentGrobidXML fails to extract metadata objects
MPImportServiceContentGrobidXML
should not throw errors if an abstract or authors have not been extracted (and are therefore not present in the generated TEI XML response).
It is reasonable to expect that some source documents cannot be heuristically parsed by GROBID (or may even lack the metadata altogether). These are not valid scenarios for an error to be thrown. The existing manuscript content will be deferred to instead.