config: use config_error_nonbool() instead of custom messages

config: use config_error_nonbool() instead of custom messages

A few config callbacks use their own custom messages to report an
unexpected implicit bool like:

  [merge "foo"]
  driver

These should just use config_error_nonbool(), so the user sees
consistent messages.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>