pool: Fix ISE in CacheEntryImpl#toString

Fixes

18 Sep 2014 12:57:38 (pdc_kth_se_034) [admin] Uncaught exception in thread sweeper-free
java.lang.IllegalStateException: Attribute is not defined: STORAGEINFO
        at org.dcache.vehicles.FileAttributes.guard(FileAttributes.java:150) ~[dcache-core-2.8.4.jar:2.8.4]
        at org.dcache.vehicles.FileAttributes.getStorageInfo(FileAttributes.java:296) ~[dcache-core-2.8.4.jar:2.8.4]
        at org.dcache.pool.repository.v5.CacheEntryImpl.toString(CacheEntryImpl.java:139) ~[dcache-core-2.8.4.jar:2.8.4]
        at java.lang.String.valueOf(String.java:2854) ~[na:1.7.0_51]
        at java.lang.StringBuilder.append(StringBuilder.java:128) ~[na:1.7.0_51]
        at org.dcache.pool.classic.SpaceSweeper2.reclaim(SpaceSweeper2.java:335) ~[dcache-core-2.8.4.jar:2.8.4]
        at org.dcache.pool.classic.SpaceSweeper2.access$000(SpaceSweeper2.java:39) ~[dcache-core-2.8.4.jar:2.8.4]
        at org.dcache.pool.classic.SpaceSweeper2$1.run(SpaceSweeper2.java:209) ~[dcache-core-2.8.4.jar:2.8.4]
Waiting for data... (interrupt to abort)

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/7368/
(cherry picked from commit c91e19508b7c32999c8ffc90ada0f1235ccf2dfc)