xjs: display filename when unable to read a schema file

[RayPlante]

When using the xjs validater from the command line and a schema file cannot be read (e.g. file not found), the command exits with an "Unexpected error:" with only the IOError message; no file name is given. This is not enough information for the user to figure out how to fix the problem.

More information should be provided, with the filename being the minimum. It would be better to handle this error explicitly indicating, providing as much information as possible.

This particular IOError occurs in xjs.schemaloader.load_schema().