DRILL-5056: Fix UserException to write full message to log
A case occurred in which the External Sort failed during spilling.
All that was written to the log was:
2016-11-18 ... INFO o.a.d.e.p.i.xsort.ExternalSortBatch - User Error Occurred
Modified the logging code to provide more information to aid tracking down problems that occur in the field.
closes #665
