dcap: ensure that we remove session on DoorTransferFinishedMessage

does not fix, but ensure that we do cleanup in case of:

java.lang.NullPointerException: null
        at diskCacheV111.doors.DCapDoorInterpreterV3$SessionHandler.sendReply(DCapDoorInterpreterV3.java:963) ~[dcach
e-dcap-2.8.4.jar:2.8.4]
        at diskCacheV111.doors.DCapDoorInterpreterV3$IoHandler.doorTransferArrived(DCapDoorInterpreterV3.java:2398) ~
[dcache-dcap-2.8.4.jar:2.8.4]
        at diskCacheV111.doors.DCapDoorInterpreterV3.messageArrived(DCapDoorInterpreterV3.java:2695) ~[dcache-dcap-2.
8.4.jar:2.8.4]
        at diskCacheV111.doors.DCapDoor.messageArrived(DCapDoor.java:354) ~[dcache-dcap-2.8.4.jar:2.8.4]
        at dmg.cells.nucleus.CellAdapter.messageArrived(CellAdapter.java:994) ~[cells-2.8.4.jar:2.8.4]
        at dmg.cells.nucleus.CellNucleus$DeliverMessageTask.innerRun(CellNucleus.java:1070) ~[cells-2.8.4.jar:2.8.4]
        at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:975) ~[cells-2.8.4.jar:2.8.4]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_55]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_55]
        at dmg.cells.nucleus.CellNucleus$1.run(CellNucleus.java:687) [cells-2.8.4.jar:2.8.4]
        at java.lang.Thread.run(Thread.java:745) [na:1.7.0_55]
21 May 2014 04:42:04 (DCap-dcache-door-cms19-<unknown>-57629) [] Uncaught exception in thread DCap-dcache-door-cms19-
<unknown>-57629-0

kills active transfers page with accumulated transfers

Acked-by: Paul Millar
Target: master, 2.8, 2.9, 2.7, 2.6
Require-book: no
Require-notes: no
dcap: do not call toString() on error object

use String.valueOf() to handle null object as well.

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: no