Create proper dialogs when failing at contact creation

Follow-up to !28 (merged).

added Sponsor 7 label

mentioned in merge request !28 (merged)

mentioned in issue #82 (closed)

marked this issue as related to #82 (closed)

changed milestone to %Desktop 0.1.0

assigned to @sebkur

added S: in progress label

mentioned in merge request !107 (merged)

removed S: in progress label

added S: in code review label

mentioned in issue #237

closed with merge request !107 (merged)