re-word Preflight Fail msg, attempt 2

My recent PR exceeded the 50 char limit (apparently the last character must be null?).

This new attempt changes: "Preflight Fail: Horizontal position estimate Error"  to: "Preflight Fail: Position estimate Error"

In fact, this newest wording is more easily understood by the average user and is actually more consistent with the other COM_ARM_EKF_xxx related errors:

COM_ARM_EKF_HGT --> "Preflight Fail: Height estimate Error"
COM_ARM_EKF_VEL --> "Preflight Fail: Velocity estimate Error"
COM_ARM_EKF_POS --> "Preflight Fail: Position estimate Error"

src/modules/commander/PreflightCheck.cpp

@@ -555,7 +555,7 @@ static bool ekf2Check(orb_advert_t *mavlink_log_pub, vehicle_status_s &vehicle_s
 
 	if (status.pos_test_ratio > test_limit) {
 		if (report_fail) {
-			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Horizontal position estimate Error");
+			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Position estimate Error");
 		}
 
 		success = false;