2d9885c introduced some regressions. This addresses those.
* Don't throw return type notice or session write failure when in an exception
* Fix tests to properly return true/false since null is no longer falsy/successy
* Rerecord a few tests to accomodate difference in raised warnings
Creates a new SessionIdInterface and moves create_sid() into it, so existing
handlers implementing SessionHandlerInterface don't require create_sid().
SessionHandler still includes the method so the default mod can be called, but
now implements both interfaces.
Also added several more tests for this feature.