[6.x] Return the fallback when reading a file from a disk that throws - #15185

This pull request fixes an issue where a missing asset meta file would break the asset browser on any disk configured with 'throw' => true, with Unable to read file from location: .meta/example.jpg.yaml.

This was happening because Statamic\Filesystem\AbstractAdapter::get() only caught Illuminate\Contracts\Filesystem\FileNotFoundException, which Storage disks stopped throwing in Laravel 9.

On a disk with throw enabled, Laravel rethrows League\Flysystem\UnableToReadFile instead, so the exception escaped get() rather than returning the fallback. Asset::meta() relies on that fallback to know it needs to generate a missing meta file, so meta was never generated and the whole listing went down. It's not S3-specific — it affects any disk with throw enabled, which is why it's also been reported on SFTP.

This PR fixes it by catching UnableToReadFile as well. The FileNotFoundException catch has to stay: AbstractAdapter is shared by FlysystemAdapter, which wraps a Storage disk, and FilesystemAdapter, which wraps Illuminate\Filesystem\Filesystem — and the latter still throws it today.

There are two routes to the same failed read, and this PR covers both with tests:

• Browsing assets that never had meta (uploaded directly to the disk) hits it via Asset::meta()
• Uploading while the last_modified index is in use hits it via the indexer, which calls lastModified() during $store->save() — before AssetRepository::save() writes the meta file.

Fixes #8641
Fixes #11961
Related: #11966