Moved get_serializer() call in dumpdata command.

Moved the get_serializer() call within the condition that checks public
serializers.  This will allow exceptions other than
SerializerDoesNotExist to be raised in order to provide the caller with
useful information, e.g when pyyaml is not installed.

django/core/management/commands/dumpdata.py

@@ -106,11 +106,11 @@ class Command(BaseCommand):
         # Check that the serialization format exists; this is a shortcut to
         # avoid collating all the objects and _then_ failing.
         if format not in serializers.get_public_serializer_formats():
-            raise CommandError("Unknown serialization format: %s" % format)
+            try:
+                serializers.get_serializer(format)
+            except serializers.SerializerDoesNotExist:
+                pass
 
-        try:
-            serializers.get_serializer(format)
-        except KeyError:
             raise CommandError("Unknown serialization format: %s" % format)
 
         def get_objects():