Can't save UML file - support for custom types

[YummyHOF]

1. Create UML class
2. Add attribute
3. Add custom type to the attribute

4. Attempt to save

I get the error, that the key [name of custom type] wasn't found.

[JanBliznicenko]

Hello, yes, generating types that are not predefined UML primitive types or parts of the model is something that needs to be done: #21

For now, it would be nice to at least warn the user why does it happen.