stylix: apply standardized message convention (#1155)

Apply the standardized message convention, introduced in commit
5e8be7521e ("treewide: simplify and standardize message convention
(#796)").

Fixes: c8e4a0d218 ("treewide: optionalize stylix.image option (#717)")
Link: https://github.com/danth/stylix/pull/1155

Reviewed-by: awwpotato <153149335+awwpotato@users.noreply.github.com>

stylix/palette.nix

@@ -169,7 +169,10 @@ in
     assertions = lib.mkIf cfg.enable [
       {
         assertion = !(cfg.image == null && cfg.base16Scheme == null);
-        message = "One of `stylix.image` or `stylix.base16Scheme` must be set";
+
+        message = ''
+          stylix: one of `stylix.image` or `stylix.base16Scheme` must be set
+        '';
       }
     ];