Update sortOrder message

[NikolayFrantsev]

Hey.

Current message of sortOrder check is useless, since it's hard to understand correct properties order.
I updated message with useful hint to help ordering properties.

[SimenB]

@Shutnik Could you add a test for this?

It's mostly to ensure I don't break it when refactoring later 😄

[SimenB]

Missing test

[NikolayFrantsev]

@SimenB I checked /test/test.js and seems there are no examples for testing message formats. Can you please suggest me way to write tests for current case?

[SimenB]

@Shutnik Easiest would be to spy on app.msg and assert it was called with the correct message.

I threw together an example in the v2 branch for you.

5e373b8

If you wanna make the PR out to v2 instead, I would prefer that 😄 Both works, though! If you don't want to, you can spy and assert with Sinon on the master branch.

I'm currently rewriting how rules report (#356), after which tests for messages will be much cleaner.