fix specs for rails3 to expect a 404 response instead of catching an exception

Change-Id: I7af57c4fcae02f1ba5c0fe5e446a0b6229bc0ca4
Reviewed-on: https://gerrit.instructure.com/29796
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>