IMPALA-919: Clarify error message for bad explain levels
IMPALA-919: Clarify error message for bad explain levels

Change-Id: I1c446ec75578343446d9f5c989467c294cba0ca7
(cherry picked from commit 00e6e0cb6b54b2056a7014323ffec21a94538ce1)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5525
Reviewed-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>
Tested-by: jenkins

XiaominZhang/Impala,bratatidas9/Impala-1,cloudera/recordservice,cloudera/recordservice,brightchen/Impala,gerashegalov/Impala,theyaa/Impala,lnliuxing/Impala,bowlofstew/Impala,henryr/Impala,brightchen/Impala,rdblue/Impala,tempbottle/Impala,caseyching/Impala,cgvarela/Impala,placrosse/ImpalaToGo,gerashegalov/Impala,bowlofstew/Impala,XiaominZhang/Impala,scalingdata/Impala,cgvarela/Impala,lirui-intel/Impala,ImpalaToGo/ImpalaToGo,placrosse/ImpalaToGo,grundprinzip/Impala,grundprinzip/Impala,lnliuxing/Impala,cgvarela/Impala,cchanning/Impala,bowlofstew/Impala,bowlofstew/Impala,caseyching/Impala,bratatidas9/Impala-1,tempbottle/Impala,ImpalaToGo/ImpalaToGo,XiaominZhang/Impala,bratatidas9/Impala-1,bowlofstew/Impala,tempbottle/Impala,cchanning/Impala,tempbottle/Impala,gerashegalov/Impala,lirui-intel/Impala,caseyching/Impala,placrosse/ImpalaToGo,cloudera/recordservice,rdblue/Impala,henryr/Impala,henryr/Impala,lirui-intel/Impala,cloudera/recordservice,XiaominZhang/Impala,placrosse/ImpalaToGo,caseyching/Impala,ImpalaToGo/ImpalaToGo,henryr/Impala,bratatidas9/Impala-1,grundprinzip/Impala,kapilrastogi/Impala,brightchen/Impala,kapilrastogi/Impala,gerashegalov/Impala,henryr/Impala,lnliuxing/Impala,lnliuxing/Impala,theyaa/Impala,cchanning/Impala,XiaominZhang/Impala,kapilrastogi/Impala,cgvarela/Impala,cchanning/Impala,kapilrastogi/Impala,tempbottle/Impala,rdblue/Impala,rdblue/Impala,grundprinzip/Impala,placrosse/ImpalaToGo,brightchen/Impala,scalingdata/Impala,cloudera/recordservice,rdblue/Impala,brightchen/Impala,cchanning/Impala,rdblue/Impala,lnliuxing/Impala,cchanning/Impala,ImpalaToGo/ImpalaToGo,gerashegalov/Impala,brightchen/Impala,theyaa/Impala,ImpalaToGo/ImpalaToGo,scalingdata/Impala,theyaa/Impala,grundprinzip/Impala,gerashegalov/Impala,caseyching/Impala,bratatidas9/Impala-1,tempbottle/Impala,rdblue/Impala,lirui-intel/Impala,XiaominZhang/Impala,lirui-intel/Impala,cchanning/Impala,cloudera/recordservice,placrosse/ImpalaToGo,scalingdata/Impala,tempbottle/Impala,theyaa/Impala,scalingdata/Impala,gerashegalov/Impala,lirui-intel/Impala,cloudera/recordservice,kapilrastogi/Impala,ImpalaToGo/ImpalaToGo,kapilrastogi/Impala,brightchen/Impala,cgvarela/Impala,caseyching/Impala,bratatidas9/Impala-1,theyaa/Impala,bowlofstew/Impala,theyaa/Impala,scalingdata/Impala,caseyching/Impala,XiaominZhang/Impala,cgvarela/Impala,bratatidas9/Impala-1,henryr/Impala,cgvarela/Impala,kapilrastogi/Impala,grundprinzip/Impala,lirui-intel/Impala,lnliuxing/Impala,bowlofstew/Impala,lnliuxing/Impala