Allow DocumentPreProcessor.parseXML to error

[msimmer]

Expected Behavior

The parseXML in the DocumentPreProcessor class calls removeScriptElements, appendStyleSheets and appendScripts. Each of those methods should report errors should any be encountered. See here

Actual Behavior

Errors are not handled at all. The err variable is set to null and passed into callback.

Suggested Fix(es)

removeScriptElements, appendStyleSheets and appendScripts should be follow the callback pattern and report failure after each one is executed.

removeScriptElements(err => {
  if (err) {}
  appendStyleSheets(err2 => {
    // etc
  }
}