Fixes issue where certain errors would not show to the user
due to extra navigation jumps which lost the error message
in the process.
This simplifies and aligns exceptions with more directly
handled exception usage at the controller level.
Fixes#3264
As per #3047.
Also made some SAML specific fixes:
- IDP initiated login was broken due to forced default session value.
Double checked against OneLogin lib docs that this reverted logic was fine.
- Changed how the saml login flow works to use 'withoutMiddleware' on
the route instead of hacking out the session driver. This was due to
the array driver (previously used for the hack) no longer being
considered non-persistent.