Actionable error text for missing message at update

[WULCAN]

Description

A user should get an actionable error message when saving an edited translation for a message that cannot be found.

'Failed to update translation' is not a very helpful error message to the user.

Originally posted by @WULCAN in #152 (comment)

Estimated size (S, M, L)

I think this issue is Small because the resolution is very likely very local to the code setting the error message. Figuring out a good text is of course tricky, but the code change will likely be small.

Open questions

Should we log something to the Lyra administrator too? There might be some information relevant to the admin, but not the user.