AP_NavEKF2: print reason for EKF2 arming failure when no GPS is available

libraries/AP_NavEKF2/AP_NavEKF2_core.cpp

@@ -334,6 +334,9 @@ bool NavEKF2_core::InitialiseFilterBootstrap(void)
 {
     // If we are a plane and don't have GPS lock then don't initialise
     if (assume_zero_sideslip() && _ahrs->get_gps().status() < AP_GPS::GPS_OK_FIX_3D) {
+        hal.util->snprintf(prearm_fail_string,
+                           sizeof(prearm_fail_string),
+                           "EKF2 init failure: No GPS lock");
         statesInitialised = false;
         return false;
     }