Swapfiles conflict when opening session in two Vim sessions

When doing `:OpenSession` in two different Vim instances I get the following warning:

```vim
E325: ATTENTION
Found a swap file by the name ...
```

Despite having the following set in my `.vimrc`.

```vim
set shortmess+=A
let g:session_autosave = 'no'
let g:session_autoload = 'no'
let g:session_lock_enabled = 0
```

Shouldn't `set shortmess+=A` suppress this warning?
