Show diff when isort or black fails

Author: snejus

tox.ini

@@ -64,6 +64,5 @@ deps =
     black==24.2.0
 skip_install = True
 commands =
-    isort beets beetsplug test --check
-    black beets beetsplug test --check
-
+    isort beets beetsplug test --check --diff
+    black beets beetsplug test --check --diff