… an unsupported type (Deuchnord)

This PR was merged into the 4.3 branch.

Discussion
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[Cache] Log a more readable message when trying to cache an unsupported type

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #29710   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        |

Improved the warning risen when trying to save something that has a non-supported type in the Simple cache.

For instance, let's say the following code:
```php
class TestCommand extends Command
{
    private $cache;

    public function __construct(CacheInterface $cache)
    {
        parent::__construct();
        $this->cache = $cache;
    }

    // ...

    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $n = $this->cache->get('n', function () {
            return function () {
                return rand(0,10);
            };
        });

        dump($n());
    }
}
```

Running this code will give the following:
```
14:32:03 WARNING   [cache] Could not save key "n" in cache: the Closure type is not supported.
0
```

Commits
-------

21ba3c0 [Cache] Log a more readable error message when saving into cache fails