Blows-ups during rendering should set 'exploded' flag on CandidateSession

[davemckain]

RuntimeExceptions thrown during assessment rendering should probably set the 'exploded' flag on the CandidateSession, as there's nothing the candidate can do to fix things at this point and the candidate would be presented with a far nicer error page than the current very long stack trace.

[davemckain]

(Would also be worth checking the handling of CandidateExceptions on state changing candidate service calls to make sure all of this joins up correctly.)

[davemckain]

Fixed in commit 2e8d9ab.