WebAPI - Improve exception handler for model validation errors

[coni2k]

Improve exception handler for EntityValidationErrors!

To test the current state:
Use "dev-all" module in main.ts, navigate to "OData" link, and use "Project - Create own" button after removing one of the mandatory fields. It returns "Internal server error" even though it should return "400 Bad request" by mentioning what is invalid with the model.