make no GPU error in CPU-only mode a little clearer

include/caffe/util/device_alternate.hpp

@@ -7,7 +7,7 @@
 
 // Stub out GPU calls as unavailable.
 
-#define NO_GPU LOG(FATAL) << "CPU-only Mode"
+#define NO_GPU LOG(FATAL) << "CPU-only Mode: cannot make GPU call."
 
 #define STUB_GPU(classname) \
 template <typename Dtype> \