Not intuitive error log if the class templates mapping does not match with ttl model

[ghost]

I think its a good idea to report it in a programmer friendly way that if you have a missing class in your graph that you are trying to map using class template, error log can be more programmer friendly.

For now it was something like this:

undefined method propogate template  for Nil:nilclass

Though it states the problem but the following could be more clear:

No such class in your graph: example.ttl

or

Class_instance_mapping does not corresponds with any class in example.ttl

Or something more clear, based on the developers understanding. #

[white-gecko]

This is a duplicate of #97