2506 - More informative error message on name collisions

Fixes #2506

This should provide the proposed error message for conflicting names including information as to how to overcome the error. I agree the error was a bit ambiguous and so I feel this is an improvement but I am more than happy to discuss as it is a bit long winded compared to some of the other error messages.

My first contribution so go easy. I didn't test the change as there was no test coverage for the error text initially. If someone could confirm that I am doing the correct thing to get the conflicting containers ID?