Clean up validation error messages

In validation code we often throw exceptions inside exception handlers, which makes Python report the exception like a problem while handling the original exception. That's not good, first because it looks like a bug in our code, and second because the validation error output on the console becomes pretty big and it's very confusing to read. So, explicitly clear up all references to the exception "context" (by using "from None" at the end of the raise statement) for all exceptions thrown inside a exception handler.

Ref. FASTMOVING-1456