Improve error message when there's no template

If there's an attempt to render non existing template, this is the message a
user gets:

    no implicit conversion of Symbol into String

With this change the update, the error message is a bit more clear:

    No such file or directory @ rb_sysopen - <template file>

Here's a link to [example busker app](https://gist.github.com/bruno-/33958941ba98b02640d6).

Btw great job working on this mini-library. A lot can be learned from it! :+1: