New user wizard: check if the directory exists

Summary:
Do not show the user an obvious error if for some reason the default
writable location does not exist.

Test Plan: The "writable directory" is not used by default if it does not exist.

Reviewers: tbaumgart

Reviewed By: tbaumgart

Subscribers: tbaumgart, wojnilowicz

Tags: KMyMoney

Differential Revision: https://phabricator.kde.org/D7902