Warn if a non-existent blog name provided for the `blog()` helper

Currently, if you call the `blog()` helper with an non-defined blog name, it would output the following error:

> undefined method `data' for nil:NilClass

That's pretty cryptic.

This PR makes it output this instead:

> Non-existent blog name provided: sectionsZ.

I'm sorry for not providing tests for this. I don't know how to write them properly. If you're so kind to introduce tests for this PR, i will gladly learn from them.