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>
Merge pull request #1 from bruno-/improve_no_template_error

Improve error message when there's no template