ACM: check the return of compass.init()

if the compass fails to initialise then don't pass it to DCM, or we
will get no yaw control. Report the init failure to the user.

ArduCopter/sensors.pde

@@ -78,10 +78,14 @@ static int32_t read_barometer(void)
 static void init_compass()
 {
 	compass.set_orientation(MAG_ORIENTATION);						// set compass's orientation on aircraft
-	dcm.set_compass(&compass);
-	compass.init();
-	compass.get_offsets();					// load offsets to account for airframe magnetic interference
-	compass.null_offsets_enable();
+	if (!compass.init()) {
+        // make sure we don't pass a broken compass to DCM
+        Serial.println_P(PSTR("COMPASS INIT ERROR"));
+        return;
+    }
+    dcm.set_compass(&compass);
+    compass.get_offsets();					// load offsets to account for airframe magnetic interference
+    compass.null_offsets_enable();
 }
 
 static void init_optflow()