fix(frontends/lean/elaborator): instantiate metavariables before displaying error message
fix(frontends/lean/elaborator): instantiate metavariables before displaying error message

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

digama0/lean,levnach/lean,soonhokong/lean-windows,fgdorais/lean,Kha/lean,digama0/lean,htzh/lean,fpvandoorn/lean2,soonhokong/lean-osx,c-cube/lean,soonhokong/lean-osx,sp3ctum/lean,soonhokong/lean-osx,dselsam/lean,htzh/lean,eigengrau/lean,leanprover/lean,johoelzl/lean,fpvandoorn/lean,fpvandoorn/lean,rlewis1988/lean,digama0/lean,sp3ctum/lean,c-cube/lean,leanprover/lean,sp3ctum/lean,leanprover/lean,leanprover-community/lean,UlrikBuchholtz/lean,leodemoura/lean,levnach/lean,javra/lean,fpvandoorn/lean2,dselsam/lean,soonhokong/lean-windows,Kha/lean,johoelzl/lean,soonhokong/lean,leanprover-community/lean,johoelzl/lean,fpvandoorn/lean2,fgdorais/lean,johoelzl/lean,eigengrau/lean,avigad/lean,UlrikBuchholtz/lean,fgdorais/lean,c-cube/lean,avigad/lean,fpvandoorn/lean,Kha/lean,leanprover-community/lean,javra/lean,digama0/lean,htzh/lean,rlewis1988/lean,fpvandoorn/lean,leanprover-community/lean,Kha/lean,javra/lean,dselsam/lean,javra/lean,leanprover-community/lean,rlewis1988/lean,leodemoura/lean,eigengrau/lean,UlrikBuchholtz/lean,dselsam/lean,soonhokong/lean,avigad/lean,soonhokong/lean-windows,c-cube/lean,johoelzl/lean,leodemoura/lean,htzh/lean,soonhokong/lean-osx,fgdorais/lean,fpvandoorn/lean2,fgdorais/lean,sp3ctum/lean,rlewis1988/lean,fpvandoorn/lean,leodemoura/lean,sp3ctum/lean,leodemoura/lean,levnach/lean,johoelzl/lean,avigad/lean,levnach/lean,avigad/lean,leanprover-community/lean,fpvandoorn/lean,fpvandoorn/lean2,leanprover/lean,UlrikBuchholtz/lean,avigad/lean,digama0/lean,dselsam/lean,htzh/lean,soonhokong/lean-windows,leanprover/lean,eigengrau/lean,rlewis1988/lean,soonhokong/lean,soonhokong/lean-osx,soonhokong/lean,Kha/lean,dselsam/lean,leodemoura/lean,Kha/lean,leanprover/lean,c-cube/lean,javra/lean,soonhokong/lean,soonhokong/lean-windows,eigengrau/lean,UlrikBuchholtz/lean,levnach/lean,fgdorais/lean,digama0/lean,rlewis1988/lean