reduced use of mixed capitalization

e.g. 
Preflight Fail: Height estimate error
Preflight Fail: Velocity estimate error
Preflight Fail: Position estimate error
Preflight Fail: Yaw estimate error

src/modules/commander/PreflightCheck.cpp

@@ -531,7 +531,7 @@ static bool ekf2Check(orb_advert_t *mavlink_log_pub, vehicle_status_s &vehicle_s
 
 	if (status.hgt_test_ratio > test_limit) {
 		if (report_fail) {
-			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Height estimate Error");
+			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Height estimate error");
 		}
 
 		success = false;
@@ -543,7 +543,7 @@ static bool ekf2Check(orb_advert_t *mavlink_log_pub, vehicle_status_s &vehicle_s
 
 	if (status.vel_test_ratio > test_limit) {
 		if (report_fail) {
-			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Velocity estimate Error");
+			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Velocity estimate error");
 		}
 
 		success = false;
@@ -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: Position estimate Error");
+			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Position estimate error");
 		}
 
 		success = false;
@@ -567,7 +567,7 @@ static bool ekf2Check(orb_advert_t *mavlink_log_pub, vehicle_status_s &vehicle_s
 
 	if (status.mag_test_ratio > test_limit) {
 		if (report_fail) {
-			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Yaw estimate Error");
+			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Yaw estimate error");
 		}
 
 		success = false;