Make expectionCode of Promise have newline character automatically

In the `fill` method, it will check if the exception code is empty
string or has newline character in the end of string or not. However, we
didn't do any change to exceptionCode when type is Promise. Thus, we add
the newline character to make it pass the checking in `fill` method.

See more detail from the log of IRC: https://mozilla.logbot.info/servo/20180501#c14692647

components/script/dom/bindings/codegen/CodegenRust.py

@@ -6207,7 +6207,7 @@ class CGDictionary(CGThing):
                                          descriptorProvider,
                                          isMember="Dictionary",
                                          defaultValue=member.defaultValue,
-                                         exceptionCode="return Err(());"))
+                                         exceptionCode="return Err(());\n"))
             for member in dictionary.members]
 
     def define(self):