fix: add guard to if & unless helpers

fixes #1548

- add a guard to show readable syntax error for if / unless helper
- prevents against 3 different errors that can be generated by different systax errors

lib/handlebars/helpers/if.js

@@ -1,8 +1,10 @@
-import {isEmpty, isFunction} from '../utils';
+import { isEmpty, isFunction } from '../utils';
+import Exception from '../exception';
 
 export default function(instance) {
   instance.registerHelper('if', function(conditional, options) {
     if (isFunction(conditional)) { conditional = conditional.call(this); }
+    if (arguments.length != 2) { throw new Exception('#if requires exactly one argument');}
 
     // Default behavior is to render the positive path if the value is truthy and not empty.
     // The `includeZero` option may be set to treat the condtional as purely not empty based on the
@@ -15,6 +17,7 @@ export default function(instance) {
   });
 
   instance.registerHelper('unless', function(conditional, options) {
+    if (arguments.length != 2) { throw new Exception('#unless requires exactly one argument');}
     return instance.helpers['if'].call(this, conditional, {fn: options.inverse, inverse: options.fn, hash: options.hash});
   });
 }

spec/helpers.js

@@ -767,29 +767,37 @@ describe('helpers', function() {
     });
   });
 
-  describe('malformed if', function() {
+  describe('malformed if/unless', function() {
+    it('if with too few arguments, string', function() {
+      var template = CompilerContext.compile('{{#if}}{{/if}}');
+        shouldThrow(function() {
+
+            template({});
+        }, undefined, /#if requires exactly one argument/);
+    });
+
     it('if with too many arguments, string', function() {
       var template = CompilerContext.compile('{{#if test "string"}}{{/if}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'includeZero' of undefined/);
+        }, undefined, /#if requires exactly one argument/);
     });
 
-    it('if with too many arguments, undefined', function() {
-      var template = CompilerContext.compile('{{#if test undefined}}{{/if}}');
+    it('unless with too many arguments, undefined', function() {
+      var template = CompilerContext.compile('{{#unless test undefined}}{{/unless}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'hash' of undefined/);
+        }, undefined, /#unless requires exactly one argument/);
     });
 
     it('if with too many arguments, null', function() {
       var template = CompilerContext.compile('{{#if test null}}{{/if}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'hash' of null/);
+        }, undefined, /#if requires exactly one argument/);
     });
   });
 });