Better error catching for paletteer_d()

[EmilHvitfeldt]

Just like here

paletteer/R/paletteer_c.R

Lines 19 to 24 in c070482

	if (!(palette %in% c_names)) {
	stop(
	"Palette not found. Make sure both package and palette ",
	'name are spelled correct in the format "package::palette"'
	)
	}