check for autograde files should happen before tango_upload

[cg2v]

If the autograder files are missing but autograding is configured, I think there should be some warning for students and/or instructors that there is a problem, before you see Donkey Kong after submitting a handin.

possibilities include:

• don't show the handin button if the files are missing
• don't allow autograding to be enabled if the files are missing (but files can always be lost/moved later, so this isn't perfect by itself)
• put some notice somewhere the instructors will see that there is a course with autograding enabled that won't work

[devanshk]

This is a great idea, it'll definitely improve UX.