BURs: add processing and failed states.

When a BUR is approved, put it in a `processing` state. After it
successfully finishes processing, put it in the `approved` state. If it
fails processing, put it in the `failed` state.

If approving the BUR fails with a validation error, for example because
the alias already exists or an implication lacks a wiki, then leave the
BUR in the `pending` state. The `failed` state is only for unexpected
errors during processing.

app/logical/bulk_update_request_processor.rb

@@ -171,6 +171,11 @@ class BulkUpdateRequestProcessor
         raise Error, "Unknown command: #{command}"
       end
     end
+
+    bulk_update_request.update!(status: "approved")
+  rescue StandardError
+    bulk_update_request.update!(status: "failed")
+    raise
   end
 
   # The list of tags in the script. Used for search BURs by tag.