Silence log messages commonly associated with client disconnects

When an FTP client disconnects prematurely, the FTP door deletes the incomplete
file and log the disconnect. Other services log errors too due to the missing
file and missing door:

06 May 2014 12:57:38 (PnfsManager) [extraload_ndgf_org_001 PnfsFlag 00006E16928BADB447EE8B10BDB9F90D47A4] Exception in updateFlag: CacheException(rc=10001;msg=No such file or directory: 00006E16928BADB447EE8B10BDB9F90D47A4)
06 May 2014 12:57:38 (SrmSpaceManager) [door:GFTP-bunsen-5413@dCacheDomain:1399373677151 extraload_ndgf_org_001 DoorTransferFinished 00006E16928BADB447EE8B10BDB9F90D47A4] Failed to forward message: {uoid=<1399373852975:39246>;path=[*@dCacheDomain:PoolManager@dCacheDomain:SrmSpaceManager@dCacheDomain:>GFTP-bunsen-5413@dCacheDomain];msg=GFTP-bunsen-5413@dCacheDomain not found}

These are expected failurs in case of a disconnected FTP client. Since the door
already logged the fact, there is no reason for other services to log these
errors.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6971/