git-gui: Ensure error dialogs always appear over all other windows

git-gui: Ensure error dialogs always appear over all other windows

If we are opening an error dialog we want it to appear above all of
the other windows, even those that we may have opened with a grab
to make the window modal.  Failure to do so may allow an error
dialog to open up (and grab focus!) under an existing toplevel,
making the user think git-gui has frozen up and is unresponsive,
as they cannot get to the dialog.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>