Teachers can see the locked exercises, but can't download them

[Salmela]

Teatchers get the locked exercises from Course.getExercises method, but they can't download them.
Students won't get the locked exercises from getExercises method.
Admins can see all the exercises and download them.

It would be nice if there would be some way to tmc-clients to know if they can download exercise before they try to download them. Or remove the locked exercises from the exercise list if the user can't download them.