AP_Compass: report compass initialisation failure

don't just put out rubbish values

git-svn-id: https://arducopter.googlecode.com/svn/trunk@2717 f9c3cf11-9bcb-44bc-f272-b75c42450872

libraries/AP_Compass/examples/AP_Compass_test/AP_Compass_test.pde

@@ -25,7 +25,10 @@ void setup()
   Serial.begin(115200);
   Serial.println("Compass library test (HMC5843 and HMC5883L)");
   Wire.begin();
-  compass.init();	 // Initialization
+  if (!compass.init()) {
+	  Serial.println("compass initialisation failed!");
+	  while (1) ;
+  }
 
   compass.set_orientation(AP_COMPASS_COMPONENTS_DOWN_PINS_FORWARD);  // set compass's orientation on aircraft.
   compass.set_offsets(0,0,0);  // set offsets to account for surrounding interference