Ensure that planner warnings get shown before calculations are run

Linus pointed out that the warning wasn't shown until the main loop got
control back, so even a gtk_widget_show_all() doesn't really help to make
sure that things are shown right then.

This commit adds a little loop to handle all pending gtk_events before
exiting the show_error() function. Now the warning should be shown BEFORE
a potentially slow calculation gets started.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>