A name nothing covers raised `KeyError` in four places and
`AmbiguousDatabaseError` in a fifth, so a caller had to catch both and neither
name said what happened. `UnknownDatabaseError` is all of them, exported from
`haiku.rag.store` beside the other errors.
It subclasses `KeyError`, since selecting by name is a lookup, and prints its
message plainly rather than quoted as a missing key. Both CLIs turn it into the
same clean exit they already gave the others.
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