Add location to the "Rule ... is already defined" error message

See this discussion:

  eb5875bc6a (commitcomment-17970220)

lib/compiler/passes/report-duplicate-rules.js

@@ -11,12 +11,14 @@ function reportDuplicateRules(ast) {
     rule: function(node) {
       if (rules.hasOwnProperty(node.name)) {
         throw new GrammarError(
-          "Rule \"" + node.name + "\" is already defined.",
+          "Rule \"" + node.name + "\" is already defined "
+            + "at line " + rules[node.name].start.line + ", "
+            + "column " + rules[node.name].start.column + ".",
           node.location
         );
       }
 
-      rules[node.name] = true;
+      rules[node.name] = node.location;
     }
   });
 

spec/unit/compiler/passes/report-duplicate-rules.spec.js

@@ -10,7 +10,7 @@ describe("compiler pass |reportDuplicateRules|", function() {
       'start = "a"',
       'start = "b"'
     ].join('\n'), {
-      message:  'Rule "start" is already defined.',
+      message:  'Rule "start" is already defined at line 1, column 1.',
       location: {
         start: { offset: 12, line: 2, column:  1 },
         end:   { offset: 23, line: 2, column: 12 }