Fix error message in impala-shell for queries that crash the impalad

An anti-join query in IMPALA-1175 revealed that the shell
mishandled the exception when the impalad would crash. This
fix prints the correct error message and sets the shell
state as disconnected.

Note there isn't a way to test this easily due to the difficulty
in coming up with a query that will crash the impalad.

Change-Id: I534674880db224a0d93dfd8bd2c081a12b65532b
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3998
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins