get_courses doesn't return ErrorDescriptor

common/lib/xmodule/xmodule/modulestore/mongo/base.py

@@ -557,7 +557,7 @@ class MongoModuleStore(ModuleStoreWriteBase):
         '''
         Returns a list of course descriptors.
         '''
-        return sum(
+        base_list = sum(
             [
                 self._load_items(
                     SlashSeparatedCourseKey(course['_id']['org'], course['_id']['course'], course['_id']['name']),
@@ -574,6 +574,7 @@ class MongoModuleStore(ModuleStoreWriteBase):
             ],
             []
         )
+        return [course for course in base_list if not isinstance(course, ErrorDescriptor)]
 
     def _find_one(self, location):
         '''Look for a given location in the collection.  If revision is not

common/lib/xmodule/xmodule/modulestore/split_mongo/split.py

@@ -70,6 +70,7 @@ from bson.objectid import ObjectId
 from xmodule.modulestore.split_mongo.mongo_connection import MongoConnection
 from xblock.core import XBlock
 from xmodule.modulestore.loc_mapper_store import LocMapperStore
+from xmodule.error_module import ErrorDescriptor
 
 log = logging.getLogger(__name__)
 #==============================================================================
@@ -313,7 +314,9 @@ class SplitMongoModuleStore(ModuleStoreWriteBase):
                 'structure': entry,
             }
             root = entry['root']
-            result.extend(self._load_items(envelope, [root], 0, lazy=True))
+            course_list = self._load_items(envelope, [root], 0, lazy=True)
+            if not isinstance(course_list[0], ErrorDescriptor):
+                result.append(course_list[0])
         return result
 
     def get_course(self, course_id, depth=None):