Revert "Refers #99 -- Use isinstance instead of try/except"

This reverts commit 03a29ef2c23bee9e437bff5ab531ae8b447d3d3b.

See https://github.com/EuroPython/djep/commit/03a29ef#commitcomment-5815783 for details
Refers #99 -- Fix reference to DoesNotExist exception