Fix ReferenceError in compiler passes.

Previously, the report-left-recursion and report-missing-rules passes
used PEG.GrammarError without requiring it, causing a ReferenceError.

Since requiring lib/peg.js would cause circular requirements, this
commit imports lib/grammar-error.js as GrammarError.

The bug was introduced in commit
4cda79951a.

Fixes GH-135.

lib/compiler/passes/report-left-recursion.js

@@ -1,4 +1,5 @@
-var utils = require("../../utils");
+var utils        = require("../../utils"),
+    GrammarError = require("../../grammar-error.js");
 
 /* Checks that no left recursion is present. */
 module.exports = function(ast) {
@@ -48,7 +49,7 @@ module.exports = function(ast) {
     rule_ref:
       function(node, appliedRules) {
         if (utils.contains(appliedRules, node.name)) {
-          throw new PEG.GrammarError(
+          throw new GrammarError(
             "Left recursion detected for rule \"" + node.name + "\"."
           );
         }

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

@@ -1,4 +1,5 @@
-var utils = require("../../utils");
+var utils        = require("../../utils"),
+    GrammarError = require("../../grammar-error.js");
 
 /* Checks that all referenced rules exist. */
 module.exports = function(ast) {
@@ -30,7 +31,7 @@ module.exports = function(ast) {
     rule_ref:
       function(node) {
         if (!utils.findRuleByName(ast, node.name)) {
-          throw new PEG.GrammarError(
+          throw new GrammarError(
             "Referenced rule \"" + node.name + "\" does not exist."
           );
         }