Grammar error messages 3/3: Improve the left recursion message

New wording is more explanatory.

Part of #371.

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

@@ -40,7 +40,7 @@ function reportLeftRecursion(ast) {
         visitedRules.push(node.name);
 
         throw new GrammarError(
-          "Possible left recursion detected ("
+          "Possible infinite loop when parsing (left recursion: "
             + visitedRules.join(" -> ")
             + ").",
           node.location

spec/unit/compiler/passes/report-left-recursion.spec.js

@@ -7,7 +7,7 @@ describe("compiler pass |reportLeftRecursion|", function() {
 
   it("reports direct left recursion", function() {
     expect(pass).toReportError('start = start', {
-      message:  'Possible left recursion detected (start -> start).',
+      message:  'Possible infinite loop when parsing (left recursion: start -> start).',
       location: {
         start: { offset:  8, line: 1, column:  9 },
         end:   { offset: 13, line: 1, column: 14 }
@@ -20,7 +20,7 @@ describe("compiler pass |reportLeftRecursion|", function() {
       'start = stop',
       'stop  = start'
     ].join("\n"), {
-      message:  'Possible left recursion detected (start -> stop -> start).',
+      message:  'Possible infinite loop when parsing (left recursion: start -> stop -> start).',
       location: {
         start: { offset: 21, line: 2, column:  9 },
         end:   { offset: 26, line: 2, column: 14 }