Implement a unified type assertion errors handler

[amanenk]

There are a lot of type assertions that require checks and every time developer needs to build an error by hand.
An integrated solution would ease the process of development.
First construction that comes to mind:

r, ok := resource.Item.(types.Type)
if !ok {
return fmt.Errorf("wrong type assertion: got %s instead of %s", 
  reflect.TypeOf(r).Name(),
  reflect.TypeOf(types.Type{}).Name())
}

[amanenk]

Since we tell which type of object we use when generating a code we can try to implement it in cq-gen to automatically generate such a check in fetchers and resolvers.