Share collection: Notify about invalid email addresses

[MrSerth]

Inviting someone is done by entering their email address. However, if mistyping the address, there is currently no information that nothing was sent. Since we also lack proper email messages (as described in #1413), a typo might be unnoticed.

Therefore, we should discuss whether and how we want to validate email addresses during the invitation phase and how to notify about errors. Potentially, a flash message is just enough ;).