LoadIncrementalHFiles should be more verbose after unrecoverable error

[Jan Lukavský]

Hi all,

I have come across a logging issue in LoadIncrementalHFiles which makes 
debugging a little harder. I explained the issue in

https://issues.apache.org/jira/browse/HBASE-11674

and I tried to attach a patch for it. The patch is not ideal, but could 
carry enough information. I was not able to (easily) retrieve the 
regionserver hostname from the RegionServerCallable, but maybe someone 
could improve the patch? :)

Thanks for reply,
 Jan