xrootd: Fix NPE and log suspicious exceptions

Fixes the following NPE:

java.lang.NullPointerException: null
        at org.dcache.xrootd.protocol.messages.ErrorResponse.<init>(ErrorResponse.java:35) ~[xrootd4j-1.2.0.jar:1.2.0]
        at org.dcache.xrootd.stream.ChunkedResponseWriteHandler.flush(ChunkedResponseWriteHandler.java:215) ~[xrootd4j-1.2.0.jar:1.2.0]
        at org.dcache.xrootd.stream.ChunkedResponseWriteHandler.handleDownstream(ChunkedResponseWriteHandler.java:116) ~[xrootd4j-1.2.0.jar:1.2.0]
        at org.jboss.netty.channel.Channels.write(Channels.java:704) ~[netty-3.6.5.Final.jar:na]

The patch also adds some additional logging that may help us figure out
which exception triggered the problem.

Target: master, 1.2
Patch: http://rb.dcache.org/r/5786/
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Ticket: http://rt.dcache.org/Ticket/Display.html?id=7937
Merge branch 'fix/npe-in-errormessage'