s:callback: Always return list.
s:callback: Always return list.

Fixes "E706: Variable type mismatch for: ret" when an exception is
caught.

mjwestcott/fzf,mjwestcott/fzf,janlazo/fzf,stuha/wbr-zsh,wilywampa/fzf,me-and/fzf,mjwestcott/fzf,tomfitzhenry/pkg-fzf,wilywampa/fzf,junegunn/fzf,janlazo/fzf,lululau/fzf,qiemem/fzf,tomfitzhenry/pkg-fzf,janlazo/fzf,qiemem/fzf,me-and/fzf,infokiller/fzf,qiemem/fzf,wilywampa/fzf,infokiller/fzf,junegunn/fzf,tomfitzhenry/pkg-fzf,infokiller/fzf,lululau/fzf,me-and/fzf,junegunn/fzf,lululau/fzf