Ensure all exception rethrows use EDI.

[bartdesmet]

Code got into BehaviorSubject<T> that didn't leverage IExceptionServices to perform the rethrow. This mechanism was part of the PlatformServices separation and has also been used for hosting layers. Restore this for BehaviorSubject<T>.

Also simplifying all of the use sites of ThrowIfNotNull by using ?. which works well with flow analysis and #nullable tracking (given the DoesNotReturn attribute on Throw and the lack of such an attribute that can make ThrowIfNotNull conditionally non-returning based on the nullability of the operand; DoesNotReturnIf only works with Boolean predicates).