XML parse errors are not verbose enough

[rafalcieslak]

Parsing an xml file (either a path save file, or a module collection description) can yeild a large number of possible errors. Even if they are displayed to the user, the visiible messages are not verbose enough. These messages should contain at least the line and character numbers, and rapidxml error description. Calculating the line number is not obvious, though, because it has to be performed on the original string, not modified by rapidxml parser.