BUGFIX: Set templatePathAndFilename for serverErrorExceptions to prevent "Template not found"

useCustomErrorView() of the AbstractExceptionHandler only checks if the array key
templatePathAndFilename is set, but not if it's a valid string. This means that
setting templatePathAndFilename to null will cause an exception during exception
rendering, hiding the original error.

Author: paxuclus

Configuration/Settings.yaml

@@ -7,6 +7,7 @@ Neos:
             matchingStatusCodes: [ 500 ]
             options:
               logException: true
+              templatePathAndFilename: 'resource://Neos.Flow/Private/Templates/Error/Default.html'
 
 Netlogix:
   ErrorHandler: