[BinderTransport] Add more info to class not found error msg (#28009)

src/core/ext/transport/binder/client/jni_utils.cc

@@ -46,7 +46,10 @@ jclass FindNativeConnectionHelper(JNIEnv* env) {
   gpr_log(GPR_ERROR,
           "Cannot find binder transport Java helper class. Did you invoke "
           "grpc::experimental::InitializeBinderChannelJavaClass correctly "
-          "beforehand?");
+          "beforehand? Did the APK correctly include the connection helper "
+          "class (i.e depends on build target "
+          "src/core/ext/transport/binder/java/io/grpc/binder/"
+          "cpp:connection_helper) ?");
   // TODO(mingcl): Maybe it is worth to try again so the failure can be fixed
   // by invoking this function again at a different thread.
   return nullptr;