Updated Adapter to use _mainAdapter->has()
Updated Adapter to use _mainAdapter->has()

Updated the Adapter to use `_mainAdapter->has()` instead of the original methods like `->read()` or `->getMetadata()` on the mainAdapter. Some (if not even almost all) adapters throw some kind of exception if core php methods like `SplFileInfo::getFileInfo` fail. Relying on the `->has()` functionality is much safer.
BePark/flysystem-fallback-to-read-only-adapter