Give better feedback to user about disbaling BLE

This little chunk of pre-processor logic could/should migrate into
microbit-dal in the long run.

source/examples/proximity-heart/main.cpp

@@ -41,7 +41,11 @@ DEALINGS IN THE SOFTWARE.
 #include "MicroBit.h"
 
 #if MICROBIT_BLE_ENABLED 
-#error "This example needs BLE to be disabled in the microbit config MicroBitConfig.h"
+ #ifdef YOTTA_CFG
+  #error "This example needs BLE to be disabled. Use the yotta config.json in the proximit-heart directory to do this"
+ #else
+  #error "This example needs BLE to be disabled in the microbit config file in the microbit-dal: MicroBitConfig.h"
+ #endif
 #endif
 
 MicroBit    uBit;