Add forward_variables_from() and target() to GN

tools/gn/target_generator.cc

Index: tools/gn/target_generator.cc
diff --git a/tools/gn/target_generator.cc b/tools/gn/target_generator.cc
index 6018147d4c43512b4b5e6da1ad3b1bce0eb18255..cd633f200018d9d5bda1ed6813a7a78d88c0e47a 100644
--- a/tools/gn/target_generator.cc
+++ b/tools/gn/target_generator.cc
@@ -113,8 +113,8 @@ void TargetGenerator::GenerateTarget(Scope* scope,
                                     Target::STATIC_LIBRARY, err);
     generator.Run();
   } else {
-    *err = Err(function_call, "Not a known output type",
-               "I am very confused.");
+    *err = Err(function_call, "Not a known target type",
+               "I am very confused by the target type \"" + output_type + "\"");
   }
 
   if (err->has_error())