Add assert messages and print bad argument values in asserts (#5216)

Enabled the `missing_assert_message` lint

* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Lucas Meurer <lucasmeurer96@gmail.com>

crates/epaint/src/texture_atlas.rs

@@ -88,7 +88,11 @@ impl TextureAtlas {
 
         // Make the top left pixel fully white for `WHITE_UV`, i.e. painting something with solid color:
         let (pos, image) = atlas.allocate((1, 1));
-        assert_eq!(pos, (0, 0));
+        assert_eq!(
+            pos,
+            (0, 0),
+            "Expected the first allocation to be at (0, 0), but was at {pos:?}"
+        );
         image[pos] = 1.0;
 
         // Allocate a series of anti-aliased discs used to render small filled circles: