Change the interface from a boolean error indication to a
negative error vs a non-negative protection.  For the moment
this is only interface change, not making use of the new data.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>