fix: improve error message for `initialize` with missing `Nonempty` instance

This PR improves the error message when `initialize` (or `opaque`) fails
to find an `Inhabited` or `Nonempty` instance. Previously, the error was
just "failed to synthesize Inhabited Foo", which didn't hint that
`Nonempty` would also work. The new message explains that both were
tried and suggests adding `deriving Nonempty`.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>