Tests - test for expected errors

[rlskoeser]

In addition to testing for valid responses, the tests should also test for expected errors - e.g., requesting an invalid magazine or issue should return a 404.

Add tests for any other errors that should come up, or expected redirects.

Depends on #72, and also relates to #85 since the tests need to run against a known, controlled data set for the tests to work reliably.