Errors are not all negative numbers: use is_error.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220602013401.303699-16-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>