* contrib/sly-mrepl.el (sly-mrepl--stacked-errors): New buffer-local variable.
(sly-mrepl--frozen-prompt-overlays): Remove
(:freeze-prompt, :unfreeze-prompt): Simplify.
(sly-mrepl-errored-prompt-face): New face.
(sly-mrepl--prompt-face): Decides which face based on
`sly-mrepl--stacked-errors'.