fixes issue #33

two small changes:

line 20:  pass on the error so it can be handled explicitly in the gulpfile
```
-      if(err) new Error(err);
+      if(err) cb(new Error(err));
```

line 24: if render was not successfull, the _css_ variavle is undefined and causes the Buffer allocation to fail.  So if it failed, just allocate a 0 sized buffer instead.
```
-      file.contents = new Buffer(css);
 +      //create a 0 length buffer if css is undefined due to some error
 +      file.contents = new Buffer(css || 0);
```