Stop ignoring SD hex files and improve the error when its missing.

Author: tannewt

ports/nrf/.gitignore

@@ -2,6 +2,8 @@
 #########################################################
 bluetooth/s132_nrf52_2.0.1/
 
+!bluetooth/*/*.hex
+
 # Build files
 #####################
 build-*/

ports/nrf/bluetooth/bluetooth_common.mk

@@ -28,7 +28,7 @@ define STACK_MISSING_ERROR
 ###### ERROR: Bluetooth LE Stack not found ############
 #                                                     #
 # The build target requires a Bluetooth LE stack.     #
-# $(SOFTDEV_VERSION_LONG) Bluetooth LE stack not found.      #
+# $(SOFTDEV_HEX) not found. #
 #                                                     #
 # Please run the download script:                     #
 #                                                     #