fix File::addHelper unknown helper error message

lib/6to5/transformation/file.js

@@ -297,7 +297,7 @@ File.prototype.isConsequenceExpressionStatement = function (node) {
 
 File.prototype.addHelper = function (name) {
   if (!contains(File.helpers, name)) {
-    throw new ReferenceError("unknown helper " + name);
+    throw new ReferenceError("Unknown helper " + name);
   }
 
   var program = this.ast.program;

test/api.js

@@ -17,6 +17,6 @@ suite("api", function () {
     var file = new File;
     assert.throws(function () {
       file.addHelper("foob");
-    }, /unknown declaration foob/);
+    }, /Unknown helper foob/);
   });
 });