Detect errors while parsing flag line and handle them
Detect errors while parsing flag line and handle them

Fixes #1345

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