Bugfix/SK-1099 | return status code 400 if model load goes wrong and print logger.error

[Wrede]

This pull request includes changes to improve logging and error handling in the set_initial_model method of the fedn/network/api/interface.py file. The most important changes include adding informative log messages and modifying the error response format.

Improvements to logging:

• Added an info log message when a model is being added. (fedn/network/api/interface.py)
• Added an info log message to indicate which helper is being used to load the model. (fedn/network/api/interface.py)
• Added an error log message for when an error occurs during model loading. (fedn/network/api/interface.py)

Enhancements to error handling:

• Changed the error response to include a status code and a more detailed JSON message. (fedn/network/api/interface.py)