Applied @CheckReturnValue to some classes

[eugenelesnov]

Should close #2363

As I understood, we can remove @CheckReturnValue from methods in org.mockito.Mockito after adding annotation on class

[TimvdLippe]

It seems like we are going to need to update our own test suite to accommodate for these additional errors. Other than that, yes this solution LGTM!

[eugenelesnov]

@TimvdLippe How do you think, how is it better to process errors in tests..Do I have to add local variables for situations like:

error: [CheckReturnValue] Ignored return value of method that is annotated with @CheckReturnValue
        anyObject();

Or should I add errorprone dependency to use @CanIgnoreReturnValue annotation?

[TimvdLippe]

We can assign it to an ignored local variable, like I did in 182fd7f

[TimvdLippe]

Yes this is great, thanks!