scheme: handle no wallpaper dynamic scheme

Throw actually readable error and notify

src/caelestia/utils/scheme.py

@@ -157,7 +157,19 @@ class Scheme:
 
     def _update_colours(self) -> None:
         if self.name == "dynamic":
+            try:
                 self._colours = get_colours_for_image()
+            except FileNotFoundError:
+                if self.notify:
+                    notify(
+                        "-u",
+                        "critical",
+                        "Unable to set dynamic scheme",
+                        "No wallpaper set. Please set a wallpaper via `caelestia wallpaper` before setting a dynamic scheme.",
+                    )
+                raise ValueError(
+                    "No wallpaper set. Please set a wallpaper via `caelestia wallpaper` before setting a dynamic scheme."
+                )
         else:
             self._colours = read_colours_from_file(self.get_colours_path())