Help developers by indicating undefined data in debug mode

It would be great to implement a debug mode that could be enabled by default in development mode (via `process.env` or `Meteor.isDevelopment` ?) that adds logging messages when a variable is not defined in a template.

As an example the following code
```

```
could raise console info messages on render when `title` is not defined in `Template.instance().data` or not defined in template helpers or when the helper return an undefined value

I think this feature could be helpful for developers, specially when refactoring code

Do you think it would be doable without too much effect on performance ? Maybe this could be done with custom code outside of blaze core ?

No files, tests, or entry points are identified. Start by locating Blaze's template rendering and helper/data lookup paths, then determine how a development-only debug mode could report undefined template data or helper results without affecting production behavior; done means the proposed cases produce console info messages with acceptable performance impact.

Written by the indexing model from the issue text.