Strengthen tests by checking the correct error code is returned

Thanks for adding these.

The evaluation order should be reversed, for coherence with the existing codebase and with the guidelines, as Guilherme noted yesterday: <https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Useful_Test_Failures>.

Also, you don't need to add .String() to the error codes when interpolating the error message.