silent failure if no orchestrator present

[gvwilson]

1. Load library(gradethis) but do not call gradethis::gradethis_setup().
2. Write an exercise check using grade_result().
3. Run the tutorial.

Learners can run the code for that exercise, but when they press Submit code, nothing happens. According to @schloerke this is because no orchestrator has been set up. Some sort of error message should pop up in this case telling the lesson author to initialize things.

[gadenbuie]

library(gradethis) automatically calls gradethis_setup(), so authors would now need to go through a lot of manual trouble to arrive at this situation.

That said, we should still throw an error during the pre-render step. If we arrive at a -check chunk we can check that an exercise.checker has been defined and throw an error if one does not exist.