Show errors produced when doing adaptive marking

[christianp]

There was a question which used adaptive marking to replace a variable with the value of the student's answer to a gapfill part, instead of the gap. So the replaced value was a list instead of a number, and evaluating a dependent variable threw an error.
That error was only logged to the console, so it just looked like the answer was marked as incorrect.

The part feedback should show the same kind of error message as if there's an error in the marking algorithm.