[dev-context] XML error reporting

Hello Hans, the lua XML parser works nice: no errors are tolerated at all, so the following example: <?xml version="1.0" encoding="utf-8"?> <story> <windows>my secret</mouse> </story> will fail with a friendly message, but it would be really helpful if one would know at which line in the source file the error has happened to help debugging. Btw: \xmlprocess doesn't complain if the file is not found. I spent quite some time trying to figure out what I did wrong in the definitions again, only to figure out that the file had been saved under a different location :( None of this is high-priority, but only "would-be-nice-to-have-in-distant-future" request. Thanks a lot, Mojca