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

tools/gn/parser.cc

Index: tools/gn/parser.cc
diff --git a/tools/gn/parser.cc b/tools/gn/parser.cc
index 651c0f88d5e8f0e965117764fe6f4547acbc5852..c207ce7179ec72474ad20603d0e8ab929b7f0793 100644
--- a/tools/gn/parser.cc
+++ b/tools/gn/parser.cc
@@ -388,9 +388,10 @@ scoped_ptr<ParseNode> Parser::BinaryOperator(scoped_ptr<ParseNode> left,
   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);