caddyhttp: Accept placeholders in vars matcher key

Until now, the vars matcher has unintentionally lacked parity with the
map directive: the destination placeholders of the map directive would
be expressed as placeholders, i.e. {foo}. But the vars matcher would
not use { }: vars foo value

This looked weird, and was confusing, since it implied that the key
could be dynamic, which doesn't seem helpful here.

I think this is a proper bug fix, since we're not used to accessing
placeholders literally without { } in the Caddyfile.

replacer.go

@@ -238,6 +238,8 @@ func toString(val any) string {
 		return v
 	case fmt.Stringer:
 		return v.String()
+	case error:
+		return v.Error()
 	case byte:
 		return string(v)
 	case []byte: