IMPALA-752: Improve INSERT error message for unsupported file formats

Change-Id: Ib16817d6e49d3df30643563eb9ec5573a920bba7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1911
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins