INT-3266 (S)FTP Close Dirty Cached Sessions

If an exception occurs on a session it should be physically closed
and not reused because its state is indeterminate and the next
operation might fail.

Note: The booleans within CachedSession do not need to be volatile
because it is a short-lived object only used by the current thread.

Also fixes the assertion message in RFT.get().

JIRA: https://jira.springsource.org/browse/INT-3266

INT-3266 Polishing - PR Comments

Remove need for SuppressWarnings.
Fix 2 test cases where the exception has an additional cause.

Fix invalid `GenericMessage` import
Fix `e.getCause()` in `FtpServerOutboundTests` & `SftpServerOutboundTests` tests