[checker] Better error message for bad recursive trees
[checker] Better error message for bad recursive trees

ejgallego/coq,Zimmi48/coq,gares/coq,ppedrot/coq,JasonGross/coq,letouzey/coq-wip,gares/coq,JasonGross/coq,SkySkimmer/coq,ppedrot/coq,maximedenes/coq,ppedrot/coq,SkySkimmer/coq,maximedenes/coq,letouzey/coq-wip,ejgallego/coq,Matafou/coq,letouzey/coq-wip,Matafou/coq,ejgallego/coq,letouzey/coq-wip,Zimmi48/coq,silene/coq,SkySkimmer/coq,coq/coq,ppedrot/coq,coq/coq,ppedrot/coq,SkySkimmer/coq,letouzey/coq-wip,coq/coq,JasonGross/coq,ejgallego/coq,maximedenes/coq,gares/coq,Matafou/coq,JasonGross/coq,silene/coq,Zimmi48/coq,silene/coq,gares/coq,maximedenes/coq,Zimmi48/coq,maximedenes/coq,gares/coq,coq/coq