Message is saved even if no recipient is available

[nicodh]

Followup to #6847

If a message can't be send since the recipient key is missing, the message should not be saved in database.

Current behaviour: the message is saved in state "pending"

To reproduce (in Desktop):

In a chatmail account create a group with a non verified contact

Send a message to that user

An error is shown (when deltachat/deltachat-desktop#5092 is merged )

Leave the chat and open it again

The message is shown in state "Pending"