Properly handle unavailable external storages

[PVince81]

An unavailable external storage is whenever a storage has already been mounted once, the mount point exists, but one of the following occurred:

1. Remote credentials/password changed
2. Temporary network failure, remote not reachable

So far, not all the backends properly support this and some can have weird behaviors.
The only one that seems to work correctly is the "ownCloud" backend (and server to server sharing).

Here are the backends to verify and/or raise tickets for:

• Local !
• SFTP: SFTP downloads empty files if remote password was changed #11785 (possible fix at [WIP] Use exceptions in SFTP backend #13390 )
• FTP
• Dropbox
• Google Drive
• Amazon S3
• Swift
• WebDAV
• SMB: SMB storage must throw StorageNotAvailable in specific cases #21219

We need to make sure that every backend will throw a \OCP\Files\StorageNotAvailableException whenever one of the causes mentioned above appear.

@MTRichards @craigpg @karlitschek @icewind1991 @jvillafanez @jmaciasportela

[PVince81]

The sync client also needs to be able to react to 503, this was raised here: owncloud/client#1923

[PVince81]

Note: it is the responsibility of the Storage class to decide whenever the remote error needs to lead to StorageNotAvailableException or another exception.

[PVince81]

@icewind1991 can you help with this ?

[PVince81]

Goal is that when an ext storage isn't available WebDAV and Web UI still work instead of throwing 500.
Making the ext storages all raise \OCP\Files\StorageNotAvailableException when appropriate should make it work.
Note that #11992 #11992 needs to be merged first for it to work with WebDAV.

[icewind1991]

WebDAV should throw the correct exceptions already
FTP I dont think is doable with the current backend (based on the builtin stream wrapper)
SMB might be doable with the currently backend but probably a lot of work, the new SMB backend will make it easy.
SFT, Amazon, Swift, Google Drive and Dropbox should be doable as long as their libraries expose the correct exceptions

[PVince81]

Thanks for the info.
If needed we can split the task in separate tickets and do this progressively.

From what I saw with Dropbox, if I simulate a missing "api.dropbox.com" it will throw a "NotFound" exception which might or might not be the same as a file not found exception. This means potentially extra logic to properly detect such cases.