don't include extension in template argument
don't include extension in template argument

fixes a deprecation warning

test plan:
 * force a 404 or 500, and it should still render a pretty page

Change-Id: I37c2257d6fc9fc333cdf71eb410d559e13d9fbe5
Reviewed-on: https://gerrit.instructure.com/52287
Tested-by: Jenkins
Reviewed-by: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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