Migration command doesn't error when path is incorrect

The decomposedfs migration command
```
ocis migrate decomposedfs -r .ocis/storage/users
```
does not error when a wrong path is entered. Instead it reports success.

The decomposedfs migration list command
```
ocis migrate decomposedfs -r ../.ocis/storage/users list
```
does also not error when the path is incorrect, instead it reports `pending` status for all migrations

This can be very confusing for the user as they see a succeeded migration but the list command reports something else.

Both commands should error if the given path is not a decomposedfs root.