Rephrasing two error messages for auto promotion

Using auto_promote or auto-promote can lead to confusion on using the
user facing interfaces. While auto-promote is fine for CLI it's not for
RAPI and vice-versa. This patch should eliminate this confusion.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>