After #1038 got merged, the logic for unwrapping AggregateException becomes a lot easier, as we now know that FluentAssertions doesn't wrap exceptions in an AggregateException.

Calling Throw<TException> is designed to also succeed if an exception of type TException is wrapped inside an AggregateException.
The same behavior should be present for NotThrow<TException>

This PR solves these problems.

1. NotThrow<TException> didn't look inside AggregateExceptions to see if an exception of type TException was thrown, which made it non-symmetric to Throw<TException>.
2. GetFirstNonAggregateException only looked at the InnerException of an AggregateException instead of InnerExceptions (all wrapped exceptions). E.g. it failed to handle if TException was the second InnerException.
3. As we no longer unnecessarily wrap exceptions, InterceptException unwrapped to much.