'Open the specified file in fs, and call cls.file_to_xml on it,
returning the lxml object.
Add details and reraise on error.'

@classmethod
def load_file(cls, filepath, fs, def_id):

    try:
        with fs.open(filepath) as xml_file:
            return cls.file_to_xml(xml_file)
    except Exception as err:
        msg = ('Unable    to    load    file    contents    at    path    %s    for    item    %s:    %s    ' % (filepath, def_id, err))
        raise Exception, msg, sys.exc_info()[2]
