Improve GN value origins for error messages.

tools/gn/value.cc

Index: tools/gn/value.cc
diff --git a/tools/gn/value.cc b/tools/gn/value.cc
index b85eb2d83023c3dc07ef1e7d34bb17d544e57777..df072fb8deaab5e8100727b2b6e6a2da950b2f41 100644
--- a/tools/gn/value.cc
+++ b/tools/gn/value.cc
@@ -177,7 +177,10 @@ bool Value::VerifyTypeIs(Type t, Err* err) const {
   if (type_ == t)
     return true;
 
-  *err = Err(origin(), std::string("This is not a ") + DescribeType(t) + ".");
+  *err = Err(origin(),
+             std::string("This is not a ") + DescribeType(t) + ".",
+             std::string("Instead I see a ") + DescribeType(type_) + " = " +
+             ToString(true));
   return false;
 }