add directive [build_translations]

Author: jmckenna

_static/sphinx.css

@@ -218,6 +218,10 @@ div.warning {
     background-color: #ffe4e4;
 }
 
+div.error {
+    background-color: #ffe4e4;
+}
+
 div.deprecated {
     background-color: #ffe4e4;
 }