Informative Exception for Non-Existing Markup Grammar

When a custom XmlGrammar is removed from the Document Type Model (*-doctypes.xml) but documents exist in the repository which still reference the removed XmlGrammar, this is an inconsistent configuration. An attempt to publish such a document logs a NullPointerException in the Content Management Server and publication fails. With the change, now more detail information is logged in both Content Management Server and Studio Server if a publication is attempted.

(CMS-25773)