Handle errors better when adding a contact to an issue

When a user tries to use add_contacts quick action on an issue and the action fails, success message is returned anyway. That happens eg. when an email that does not belong to any contact is used.

We should:

• [-] return an error in case no contact is added - new issue created: #349964
• don't suggest add_contacts action in case no contacts are available for the project - MR created: !77822 (merged)
• [-] add suggestions when adding a contact - follow-up issues: #350133 (closed), #350135 (closed)