Use default show_exceptions value of false

Enabling this hides exceptions during integration tests behind an HTML 500
page, which makes debugging much harder. It can also hide errors, as the test
can render a 500 and still pass.

I didn't have redis running locally, which caused one test to fail. With the
previous setting, it took me 15 minutes to find the failure. With the new
setting, it immediately prints an error about redis.