[firebase_storeage] Add check for file existence

[mozochek]

It would be nice if Reference which returns from FirestoreStorage.instance.ref('path') had some method for checking file existence. For example, Future<bool> ref.isExists(). Or make return types of getters (getData(), getDownloadURL(), getMetadata()) of Reference nullable and do not rethrow StorageException.

current console output

E/StorageException(17903): StorageException has occurred.
E/StorageException(17903): Object does not exist at location.
E/StorageException(17903):  Code: -13010 HttpResult: 404
E/StorageException(17903): {  "error": {    "code": 404,    "message": "Not Found.  Could not get object",    "status": "GET_OBJECT"  }}
E/StorageException(17903): java.io.IOException: {  "error": {    "code": 404,    "message": "Not Found.  Could not get object",    "status": "GET_OBJECT"  }}
E/StorageException(17903): 	at com.google.firebase.storage.network.NetworkRequest.parseResponse(NetworkRequest.java:434)
E/StorageException(17903): 	at com.google.firebase.storage.network.NetworkRequest.parseErrorResponse(NetworkRequest.java:451)
E/StorageException(17903): 	at com.google.firebase.storage.network.NetworkRequest.processResponseStream(NetworkRequest.java:442)
E/StorageException(17903): 	at com.google.firebase.storage.network.NetworkRequest.performRequest(NetworkRequest.java:272)
E/StorageException(17903): 	at com.google.firebase.storage.network.NetworkRequest.performRequest(NetworkRequest.java:286)
E/StorageException(17903): 	at com.google.firebase.storage.internal.ExponentialBackoffSender.sendWithExponentialBackoff(ExponentialBackoffSender.java:70)
E/StorageException(17903): 	at com.google.firebase.storage.internal.ExponentialBackoffSender.sendWithExponentialBackoff(ExponentialBackoffSender.java:62)
E/StorageException(17903): 	at com.google.firebase.storage.GetDownloadUrlTask.run(GetDownloadUrlTask.java:76)
E/StorageException(17903): 	at java.util.concurrent.ThreadPoolExecutor.processTask(ThreadPoolExecutor.java:1187)
E/StorageException(17903): 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1152)
E/StorageException(17903): 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
E/StorageException(17903): 	at java.lang.Thread.run(Thread.java:929)