Fix Exception Rendering on Job Details Template

[BobReid]

Issue

The stack trace that caused a job to fail was not displayed for failed jobs on the job details admin screen.
The html block was wrapped in an if statement checking a non-existent variable exc_info instead of job.exc_info

Fix

Update the if check to check job.exc_info instead of exc_info

[BobReid]

I am closing this out as it is caused by an incompatibility with django-rq and and old version of rq (1.7). Upgrading RQ is the real fix.