Followup 1c04d32d: release saveDialog reference even if closed

It's good that we're not trying to close the save dialog
if it's not open, but we still need to release our reference
to it in that case, otherwise trying to open the save dialog
in a new incarnation of the editor will fail.

If you opened VE, opened the save dialog, closed it,
went back to read mode, opened VE again, and tried to open the
save dialog again, it would fail because it tried to open
the cached reference to the previous editor's save dialog.

Bonus: prevent JS errors in this case by only attaching an
onClose handler if the dialog actually opened successfully.
If opening the dialog failed, the callback parameter isn't
a promise but an OO.ui.Error, and calling .always() causes
a JS error.

Change-Id: I62d6d3e7cd3df2c29a0c99e2e23bf1aac5187a12
(cherry picked from commit dad06ef49501764f428608540caa22d2daf4a4de)