Follow ups on #3648

[jnm]

PR #3648 adds some code to MockDeplyomentBackend that never gets reached by our unit tests. We should add a new test so that this code does get executed during testing.

_() isn't imported. I'll fix it, but I'll also open a new issue about adding test code that reaches these lines.
Internal discussion: https://chat.kobotoolbox.org/#narrow/stream/4-Kobo-Dev/topic/Unused.20test.20code/near/104338

Originally posted by @jnm in #3648 (comment)

Additionally, we probably shouldn't use Kobocat* stuff in the mock backend. I think it's an easy fix in this case: can these these particular exceptions be renamed, given that seems generic and not tied specifically to Kobocat? [This has been addressed in #4829]

Internal discussion: https://chat.kobotoolbox.org/#narrow/stream/4-Kobo-Dev/topic/Unused.20test.20code/near/104352