rc lint: Print an error when `lintcmd` is empty
rc lint: Print an error when `lintcmd` is empty

mawww/kakoune,Somasis/kakoune,lenormf/kakoune,jjthrash/kakoune,occivink/kakoune,casimir/kakoune,mawww/kakoune,lenormf/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,mawww/kakoune,mawww/kakoune,alexherbo2/kakoune,Somasis/kakoune,casimir/kakoune,occivink/kakoune,jkonecny12/kakoune,danr/kakoune,occivink/kakoune,casimir/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,danr/kakoune,jjthrash/kakoune,danr/kakoune,jkonecny12/kakoune,danr/kakoune,Somasis/kakoune,lenormf/kakoune,jjthrash/kakoune,casimir/kakoune,Somasis/kakoune,lenormf/kakoune,alexherbo2/kakoune,jjthrash/kakoune,occivink/kakoune