Moved error when option does not exist into a warning (this allows to improve compatibility between Coq versions).
Moved error when option does not exist into a warning (this allows to
improve compatibility between Coq versions).

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@13196 85f007b7-540e-0410-9357-904b9bb8a0f7

Lionel-Rieg/coq,letouzey/coq-wip,ejgallego/coq,JasonGross/coq,gmalecha/coq,8l/coq,gares/coq,jhjourdan/coq,TheTypoMaster/coq,Zimmi48/coq,tomprince/coq,Zimmi48/coq,letouzey/coq-wip,8l/coq,jhjourdan/coq,herbelin/coq-hh,ppedrot/coq,mattam82/coq,letouzey/coq-wip,herbelin/coq-hh,amintimany/coq,shindere/coq,mlasson/coq,barras/coq,barras/coq,aspiwack/coq,ppedrot/coq,tomprince/coq,ale-f/coq,mlasson/coq,JasonGross/coq,SkySkimmer/coq,william-lawvere/coq,ejgallego/coq,pirbo/coq,Matafou/coq,SkySkimmer/coq,TheTypoMaster/coq,gares/coq,gares/coq,maximedenes/coq,pirapira/coq2rust,Zimmi48/coq,barras/coq,TOSPIO/coq,gares/coq,ejgallego/coq,mattam82/coq,ppedrot/coq,amintimany/coq,pirapira/coq2rust,Ptival/coq,william-lawvere/coq,TOSPIO/coq,maximedenes/coq,asr/coq,gares/coq,letouzey/coq-wip,jhjourdan/coq,herbelin/github-coq,silene/coq,maximedenes/coq,mattam82/Coq-misc,TOSPIO/coq,amintimany/coq,mattam82/Coq-misc,aspiwack/coq,jpdeplaix/coq,Matafou/coq,8l/coq,herbelin/github-coq,Lionel-Rieg/coq,ppedrot/coq,gmalecha/coq,jpdeplaix/coq,coq/coq,coq/coq,ale-f/coq,maximedenes/coq,pirbo/coq,ppedrot/coq,silene/coq,TheTypoMaster/coq,aspiwack/coq,SkySkimmer/coq,ejgallego/coq,tomprince/coq,william-lawvere/coq,ale-f/coq,JasonGross/coq,herbelin/github-coq,asr/coq,silene/coq,shindere/coq,mlasson/coq,gmalecha/coq,coq/coq,asr/coq,maximedenes/coq,jpdeplaix/coq,letouzey/coq-wip,pirbo/coq,Lionel-Rieg/coq,Zimmi48/coq,Matafou/coq,Ptival/coq,mattam82/coq,Ptival/coq,SkySkimmer/coq,JasonGross/coq,shindere/coq,pirapira/coq2rust,coq/coq