Per the recommendations in qapi/error.h, return false on failure.

Use the return value in the monitor, the only place we aren't
already passing error_fatal or error_abort.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-4-richard.henderson@linaro.org>