Improve GN value origins for error messages.

tools/gn/value.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/value.h"
 
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "tools/gn/scope.h"
 
@@ skipping 159 matching lines @@
       return result;
     }
   }
   return std::string();
 }
 
 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;
 }
 
 bool Value::operator==(const Value& other) const {
   if (type_ != other.type_)
     return false;
 
   switch (type_) {
     case Value::BOOLEAN:
       return boolean_value() == other.boolean_value();
@@ skipping 15 matching lines @@
       // iteration code.
       return false;
     default:
       return false;
   }
 }
 
 bool Value::operator!=(const Value& other) const {
   return !operator==(other);
 }