Double error when closing with unsaved changes.

[jordwalke]

When you close a wintab that has not saved its changes, you'd expect to get an error (from Vim) but you get a double error which causes you to have to press enter with wintab.

[jordwalke]

I think it might be able to catch E89 and just rethrow it up.

[zefei]

Not sure if the fix of #24 resolves this issue. For exception E89: vim can catch that but unfortunately (re)throwing vim exceptions isn't allowed.

Let me know if this is still an issue after the other error is fixed.

[zefei]

Should be fixed now.