DefaultXmlFormatter: Should return plain text if formatting fails

DefaultXmlFormatter attempts to format the text returned from editors assuming it is a String representation of an XML document. However both DMN and BPMN editors allow saving invalid content i.e. plain text not being the representation of a valid XML document (in which scenarios both DMN and BPMN would have shown the ACE editor to Users). Consequentially calls to editor.getContent() may return a Promise that does not contain a String representation of an XML document. Passing this to the DefaultXmlFormatter leads to an error that is stored in the file.

Revealed during other PR review, see the comment