poolmanager: Suppress stack trace in case of unmatch units

The patch suppresses stack traces like the following:

03 Aug 2014 06:40:09 (PoolManager) [door:NFS-fax-dcap-proxy@faxDomain:1407040808696 NFS-fax-dcap-proxy PoolMgrSelectReadPool 0000D36F8581AC7043118452477B188E6EC1] Read pool selection failed
java.lang.IllegalArgumentException: Unit not matched : 0:0:0:0:0:0:0:1
        at diskCacheV111.poolManager.PoolSelectionUnitV2.match(PoolSelectionUnitV2.java:545) ~[dcache-core-2.9.6.jar:2.9.6]
        at diskCacheV111.poolManager.PoolMonitorV5$PnfsFileLocation.selectReadPool(PoolMonitorV5.java:234) ~[dcache-core-2.9.6.jar:2.9.6]
        at diskCacheV111.poolManager.RequestContainerV5$PoolRequestHandler.askIfAvailable(RequestContainerV5.java:2008) [dcache-core-2.9.6.jar:2.9.6]
        at diskCacheV111.poolManager.RequestContainerV5$PoolRequestHandler.stateEngine(RequestContainerV5.java:1488) [dcache-core-2.9.6.jar:2.9.6]
        at diskCacheV111.poolManager.RequestContainerV5$PoolRequestHandler.stateLoop(RequestContainerV5.java:1266) [dcache-core-2.9.6.jar:2.9.6]
        at diskCacheV111.poolManager.RequestContainerV5$PoolRequestHandler.access$2300(RequestContainerV5.java:691) [dcache-core-2.9.6.jar:2.9.6]
        at diskCacheV111.poolManager.RequestContainerV5$PoolRequestHandler$RunEngine.run(RequestContainerV5.java:1197) [dcache-core-2.9.6.jar:2.9.6]
        at org.dcache.util.FireAndForgetTask.run(FireAndForgetTask.java:31) [dcache-core-2.9.6.jar:2.9.6]
        at org.dcache.util.CDCExecutorServiceDecorator$1.run(CDCExecutorServiceDecorator.java:104) [dcache-core-2.9.6.jar:2.9.6]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_65]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_65]
        at java.lang.Thread.run(Thread.java:745) [na:1.7.0_65]

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7194/
(cherry picked from commit 1adf0d0b7cb9c262ab9b07009c53d35b58a6fe65)
Merge pull request #750 from gbehrmann/fix/2.6/rb7194

poolmanager: Suppress stack trace in case of unmatch units