Improve error reporting on config migration (#4210)

- **PR Description**

closes #4209

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] If a new UserConfig entry was added, make sure it can be
hot-reloaded (see
[here](https://github.com/jesseduffield/lazygit/blob/master/docs/dev/Codebase_Guide.md#using-userconfig))
* [ ] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

pkg/config/app_config.go

@@ -245,9 +245,11 @@ func migrateUserConfig(path string, content []byte) ([]byte, error) {
 
 	// Write config back if changed
 	if string(changedContent) != string(content) {
+		fmt.Println("Provided user config is deprecated but auto-fixable. Attempting to write fixed version back to file...")
 		if err := os.WriteFile(path, changedContent, 0o644); err != nil {
-			return nil, fmt.Errorf("Couldn't write migrated config back to `%s`: %s", path, err)
+			return nil, fmt.Errorf("While attempting to write back fixed user config to %s, an error occurred: %s", path, err)
 		}
+		fmt.Printf("Success. New config written to %s\n", path)
 		return changedContent, nil
 	}