"Invalid compiled template for" thrown if write to cache fails

When caching is enabled and entry is missing in cache, then smarty generates new content, stores it inside cache, and later reads it back to process it.

However, if storing to cache fails for some reason (or if the cache is cleaned before the read), smarty later throws exception

Invalid compiled template for ...

I see two problems here:

1. reading back what we just wrote to cache is resource wasting, especially if the cache is remote (redis/memcached/db)
2. caches (by definition) do not guarantee that item will be available after writing. Stored items can be purged at any time. In this context, smarty treats cache as a temporary storage, which is wrong.

Repro-code

/**
 * Simple NO-OP cache implementation. Stored data are immediately forgotten
 */
class CacheNoOP extends Smarty_CacheResource_Custom
{
    protected function fetch($id, $name, $cache_id, $compile_id, &$content, &$mtime)
    {
        $content = null;
        $mtime = null;
    }

    protected function save($id, $name, $cache_id, $compile_id, $exp_time, $content)
    {
        return false;
    }

    protected function delete($name, $cache_id, $compile_id, $exp_time)
    {
        return 0;
    }

}

$smarty = new Smarty();
$smarty->registerCacheResource('noop', new CacheNoOp());
$smarty->caching_type = 'noop';
$smarty->caching = Smarty::CACHING_LIFETIME_CURRENT;
$smarty->setCompileDir(__DIR__ . '/compile');
$smarty->setTemplateDir(__DIR__.'/tpl');

$smarty->fetch('whatever.tpl');

This is possibly related to #698

Reproduce the issue with the CacheNoOP resource and the Smarty fetch('whatever.tpl') entry point, then trace the cache miss, save, and subsequent read flow. Compare the behavior with the related issue #698; done means a failed or unavailable cache write does not produce an “Invalid compiled template” exception while processing the generated content.