Print type of offending expression in Ltac2 not-unit warning.
Print type of offending expression in Ltac2 not-unit warning.

Partial fix of #14013.

JasonGross/coq,JasonGross/coq,ejgallego/coq,JasonGross/coq,SkySkimmer/coq,Zimmi48/coq,ejgallego/coq,ejgallego/coq,coq/coq,ejgallego/coq,coq/coq,coq/coq,JasonGross/coq,Zimmi48/coq,SkySkimmer/coq,coq/coq,SkySkimmer/coq,SkySkimmer/coq,Zimmi48/coq,Zimmi48/coq