Report error locations in source file

[phfaist]

Currently, error messages give basically no information about where the error occurred in the document. Error messages should report the exact location in the document (which source file and which line) where the error was encountered.

It might be tricky to add this information because of the way latexpp processes the documents into node trees & modifies them. But we should be able to attach to all parsed latex nodes information about source file & line, and carry that information over to the modified nodes.