- Separate the failure cases into different assertions: end/default constructed vs rehashed or erased.
- Update the assertion error for AssertIsValid to not mention the end iterator case because end iterators are considered valid by AssertIsValid.
- Also fix an out-of-date comment for skip_empty_or_deleted.

PiperOrigin-RevId: 485402559
Change-Id: I593056abdc6c3565d0396fb885923fef643bf4e4