improve error message on missing helper

Via @stefanpenner
Fixes #523

dist/handlebars.js

@@ -67,7 +67,7 @@ Handlebars.registerHelper('helperMissing', function(arg) {
   if(arguments.length === 2) {
     return undefined;
   } else {
-    throw new Error("Could not find property '" + arg + "'");
+    throw new Error("Missing helper: '" + arg + "'");
   }
 });
 

dist/handlebars.runtime.js

@@ -67,7 +67,7 @@ Handlebars.registerHelper('helperMissing', function(arg) {
   if(arguments.length === 2) {
     return undefined;
   } else {
-    throw new Error("Could not find property '" + arg + "'");
+    throw new Error("Missing helper: '" + arg + "'");
   }
 });
 

lib/handlebars/base.js

@@ -44,7 +44,7 @@ Handlebars.registerHelper('helperMissing', function(arg) {
   if(arguments.length === 2) {
     return undefined;
   } else {
-    throw new Error("Could not find property '" + arg + "'");
+    throw new Error("Missing helper: '" + arg + "'");
   }
 });
 

spec/qunit_spec.js

@@ -706,7 +706,7 @@ test("if a context is not found, helperMissing is used", function() {
   shouldThrow(function() {
       var template = CompilerContext.compile("{{hello}} {{link_to world}}");
       template({});
-    }, [Error, "Could not find property 'link_to'"], "Should throw exception");
+    }, [Error, "Missing helper: 'link_to'"], "Should throw exception");
 });
 
 test("if a context is not found, custom helperMissing is used", function() {