API behavior between backends (may not be) consistent

Example on first-use of an application:

- The macos keychain backend will return an empty list and nil error the first time Keys() (or Get()) is called:

```sh
2019/06/01 07:11:51 [keyring] Considering backends: [keychain pass file]
2019/06/01 07:11:51 [keyring] Querying keychain for service="vault", keychain=""
2019/06/01 07:11:51 [keyring] Found 0 results
$ echo $?
0
```

- The libsecret backend will return a secretError with message that the keychain does not exist:

```sh
2019/06/01 07:13:25 [keyring] Considering backends: [secret-service kwallet pass file]
Error: Failed to read tokens from backend storage: The collection "vault" does not exist. Please add a key first
$ echo $?
255
```

The returned error is a generic type `secretError` and so I could handle this by wrapping keyring.List() and checking err.Error() contains the string "Please add a key first", but I am wondering what the expected behavior is for the library itself and if perhaps libsecret should be modified to not return an err if the collection does not yet exist?

I haven't checked the other backend types for how they handle first-use of Get and Keys yet.