Make error more useful with invalid XML in multi-file zip

[ibboard]

If you have a zip file with multiple XML files in it and one of them contains invalid XML then it isn't entirely clear where the problem is from the exception. For example, when using ​these data files I initially got:

WARN  [20:11:26]: FrmMainWindow.FileLoadingFinished() - Line: 249 - Failed to load /home/ibboard/Projects/dev/IBBoard.WarFoundry.GUI.GTK/bin/Debug/data/Warpath.system: Problem validating against schema for WarFoundry data: XmlSchema error: Key sequence is missing. XML  Line 57, Position 15.

This was easily locatable, as there were only two files in the zip and one wasn't 57 lines long, but in other situations this might be even more awkward.

Suggested format (if possible) is to put ";subfile-path" at the end, e.g.:

/home/ibboard/Projects/dev/IBBoard.WarFoundry.GUI.GTK/bin/Debug/data/Warpath.system;ForgeFathers.racex: Problem validating...