Log error messages when failing to parse xml from a string that contain...

Log error messages when failing to parse xml from a string that contain context of where the error occurred in the string (in definition_to_xml)

common/lib/xmodule/xmodule/raw_module.py

@@ -2,7 +2,9 @@ from pkg_resources import resource_string
 from lxml import etree
 from xmodule.mako_module import MakoModuleDescriptor
 from xmodule.xml_module import XmlDescriptor
+import logging
 
+log = logging.getLogger(__name__)
 
 class RawDescriptor(MakoModuleDescriptor, XmlDescriptor):
     """
@@ -24,4 +26,13 @@ class RawDescriptor(MakoModuleDescriptor, XmlDescriptor):
         return {'data': etree.tostring(xml_object)}
 
     def definition_to_xml(self, resource_fs):
-        return etree.fromstring(self.definition['data'])
+        try:
+            return etree.fromstring(self.definition['data'])
+        except etree.XMLSyntaxError as err:
+            lines = self.definition['data'].split('\n')
+            line, offset = err.position
+            log.exception("Unable to create xml for problem {loc}. Context: '{context}'".format(
+                context=lines[line-1][offset - 40:offset + 40],
+                loc=self.location
+            ))
+            raise