preserve path of source in log messages

[xworld21]

Currently, LaTeXML prints errors and warnings like

Error:expected:... at filename.tex; line NN col NN

but filename.tex is always just the file name, not including the location.

For the purposes of parsing the log files and linking the errors to actual files, it would be so much better if the path used to open the file were included, with some rules about relative vs absolute paths. For instance, files below the current directory should be relative, rest absolute (or without path).