Handle downloaded template sets with missing source jars

[jan-vcapgemini]

The TemplateAdapterImpl can detect template set jars which don't provide a source jar. We need to add an exception handling and maybe automatically download missing source jars if needed.

https://github.com/devonfw/cobigen/blob/28f53b0dad58ff71b30d011f0a0dfe450f646313/cobigen/cobigen-core/src/main/java/com/devonfw/cobigen/impl/adapter/TemplateAdapterImpl.java#L256-L262