Provide "debug" option when creating a Thing with a referenced WoT TM, providing error feedback

[thjaeckle]

When developing new WoT TMs and testing if the JSON skeleton created by Ditto is as expected it would be very helpful to get meaningful error messages, e.g. when:

• the model or some references submodels / referenced models cannot be retrieved
• when the TM is in invalid format / cannot be parsed
• etc.

Currently, Ditto "fails silently" and just does not create the JSON skeleton (which is also a good approach to be fault tolerant).

However, for this specific case it would make sense to configure a "strict WoT mode", e.g. via query-parameter or header field.