Fix an issue with unencrypted data bag access

- Force missing/bad data bag to throw an exception
    By accessing data bags, they will raise the intended exception and handling
    logic we wanted in the first place.

    Fixes https://github.com/rackspace-cookbooks/elkstack/issues/77.

- Add test for bad data bags behavior
    This test adds an unrelated encrypted data bag, with unencrypted lumberjack
    keypairs, reproducing the issue described in https://github.com/rackspace-cookbooks/elkstack/issues/77.