make export a bit more resilient. If there's a piece of metadata that we can't serialize - i.e. someone wrote a new bool attribute and didn't put it in the type mapping table - then we log the exception and continue.

common/lib/xmodule/xmodule/xml_module.py

@@ -379,7 +379,11 @@ class XmlDescriptor(XModuleDescriptor):
             if attr not in self.metadata_to_strip and attr not in self.metadata_to_export_to_policy:
                 val = val_for_xml(attr)
                 #logging.debug('location.category = {0}, attr = {1}'.format(self.location.category, attr))
-                xml_object.set(attr, val)
+                try:
+                    xml_object.set(attr, val)
+                except Exception, e:
+                    logging.exception('Failed to serialize metadata attribute {0} with value {1}. This could mean data loss!!!  Exception: {2}'.format(attr, val, e))
+                    pass
 
         if self.export_to_file():
             # Write the definition to a file