Improve sqlite error messages

They did not include the detailed error message, losing essential
information for troubleshooting.

Example message:

    warning: creating statement 'insert or rplace into NARs(cache, hashPart, namePart, url, compression, fileHash, fileSize, narHash, narSize, refs, deriver, sigs, ca, timestamp, present) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, 1)': at offset 10: SQL logic error, near "rplace": syntax error (in '/tmp/nix-shell.grQ6f7/nix-test/tests/binary-cache/test-home/.cache/nix/binary-cache-v6.sqlite')

It's not the best example; more important information will be in
the message for e.g. a constraint violation.

I don't see why this specific error is printed as a warning, but
that's for another commit.

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