Grammar error messages 1/3: Improve the missing rule message

New wording is more similar to wording used in various programming
languages and it is in line with the duplicate rule message.

Part of #371.

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

@@ -10,7 +10,7 @@ function reportMissingRules(ast) {
     rule_ref: function(node) {
       if (!asts.findRule(ast, node.name)) {
         throw new GrammarError(
-          "Referenced rule \"" + node.name + "\" does not exist.",
+          "Rule \"" + node.name + "\" is not defined.",
           node.location
         );
       }

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

@@ -7,7 +7,7 @@ describe("compiler pass |reportMissingRules|", function() {
 
   it("reports missing rules", function() {
     expect(pass).toReportError('start = missing', {
-      message:  'Referenced rule "missing" does not exist.',
+      message:  'Rule "missing" is not defined.',
       location: {
         start: { offset:  8, line: 1, column:  9 },
         end:   { offset: 15, line: 1, column: 16 }