Show dive import error messages in the import dialog

.. not in the main window.  And leave the import dialog open, so that
you can either try doing it again, or cancel.  This makes it much easier
to re-try a failed dive import, and actually makes the failure more
obvious too.

Todo:

 - make the "Ok" button change to "Retry" when an error happens

 - try to see if we can catch the actual status update messages from
   libdivecomputer and show them too in the import dialog.  Right now
   they are printed out to stderr by the library.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>