problem

If the error E1069 (content outside mrk tags) is produced, the filename of the affected file is missing in the output.

The reason for the error were the following characters:

Solution

Add the filename to the error message.

Make invisible other content characters visible in the error message. So is there a way to check of the "other content" is whitespace (not visible to the user?) if yes, print the code point in the error message too.