[tools][scons] Add error notice when 'EXEC_PATH' is not exist.

tools/building.py

@@ -130,6 +130,9 @@ def PrepareBuilding(env, root_directory, has_libcpu=False, remove_components = [
 
     sys.path = sys.path + [os.path.join(Rtt_Root, 'tools')]
 
+    if not os.path.exists(rtconfig.EXEC_PATH):
+        print "Error: Toolchain path (%s) is not exist, please check 'EXEC_PATH' in path or rtconfig.py." % rtconfig.EXEC_PATH
+        sys.exit(1)
     # add compability with Keil MDK 4.6 which changes the directory of armcc.exe
     if rtconfig.PLATFORM == 'armcc':
         if not os.path.isfile(os.path.join(rtconfig.EXEC_PATH, 'armcc.exe')):