Ensure that the test returns a result in various error conditions
including databases not being defined, and databases() throwing an
exception. This avoids the test timing out as a failure mode and so
saves some runtime.

Differential Revision: https://phabricator.services.mozilla.com/D31523

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1552309
gecko-commit: e47b1f64dd120de6a8f4aedff9a70e883ba885ac
gecko-integration-branch: autoland
gecko-reviewers: asuth