Report name of invalid MXL tag when converting to XML

[jhorbulyk]

When creating XML files with invalid XML tags, the name of the potentially invalid tag will not be reported. Consider this integration test. Ideally, it would be nice if the name of the invalid tag was included in the thrown error. If this PR: Leonidas-from-XIV/node-xml2js#565 is merged and then the version of xml2js is updated, then this problem will be solved.