Move error checking from render_exercise() to evaluate_exercise()

[rossellhayes]

This is the first step in the process of moving error handling code out of render_exercise() and into evaluate_exercise()

Fixes #481

[gadenbuie]

This is a great start and I really like that we'll collect all of the checking logic in one place in evaluate_exercise() rather than render_exercise().

The tests are currently written around the error checking logic being inside render_exercise() but now that we've moved that out we should

• Update the error-checking tests so they call and reference evaluate_exercise()
• Add tests the explicitly check that we get the correct expected errors from render_exercise()

[gadenbuie]

For next steps, we can update the temp directory logic so that we stay in the temp directory while evaluating the checker functions as well. So rather than with_dir() we would use setwd() and on.exit() to coordinate that step. You might also want to move the code checking logic up a bit; there's no need to do anything with the exercise temp directory until after we've performed the code checking.

[gadenbuie]

Great work, thank you!