Better handling of failed autograding

[trevorcampbell]

Right now when the autograder encounters an error, the entire grading flow quits, leaving the admin to handle cleaning up student notebooks. There are usually only a couple student notebooks with JSON errors per assignment (e.g. incorrectly merged cells, copied cells, out of date checksums, etc), but this kills the whole flow and slows down grading for everyone.

Upon encountering an autograder error, we should instead

• make the student's assignment entirely manually graded
• notify the instructor that it was an error notebook and that manual grading is required
• make sure that nbgrader knows that manual grading is required in the db

and then move along and keep grading