tools/gn: provide better error message for bogus binary expressions

tools/gn/parser.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/parser.h"
 
 #include "base/logging.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/operators.h"
 #include "tools/gn/token.h"
@@ skipping 370 matching lines @@
   if (!has_error() && !at_end())
     Consume(Token::RIGHT_BRACKET, "Expected ']'");
   return list.Pass();
 }
 
 scoped_ptr<ParseNode> Parser::BinaryOperator(scoped_ptr<ParseNode> left,
                                              Token token) {
   scoped_ptr<ParseNode> right =
       ParseExpression(expressions_[token.type()].precedence + 1);
   if (!right) {
-    *err_ =
-        Err(token,
-            "Expected right hand side for '" + token.value().as_string() + "'");
+    if (!has_error()) {
+      *err_ = Err(token, "Expected right hand side for '" +
+                             token.value().as_string() + "'");
+    }
     return scoped_ptr<ParseNode>();
   }
   scoped_ptr<BinaryOpNode> binary_op(new BinaryOpNode);
   binary_op->set_op(token);
   binary_op->set_left(left.Pass());
   binary_op->set_right(right.Pass());
   return binary_op.Pass();
 }
 
 scoped_ptr<ParseNode> Parser::IdentifierOrCall(scoped_ptr<ParseNode> left,
@@ skipping 332 matching lines @@
         break;
       }
     }
 
     // Suffix comments were assigned in reverse, so if there were multiple on
     // the same node, they need to be reversed.
     if ((*i)->comments() && !(*i)->comments()->suffix().empty())
       const_cast<ParseNode*>(*i)->comments_mutable()->ReverseSuffix();
   }
 }