Errors for new emails

[crepererum]

Error: execute command

Caused by:
    0: download mail: `https://app.tuta.com/mail/LyUY5na--F-2/NK4EWIa----9`
    1: get file infos
    2: deserialize JSON for `alloc::vec::Vec<tatutanatata::proto::messages::FileReponse>`
    3: [0]._ownerEncSessionKey: invalid type: null, expected a string at line 1 column 116

Originally posted by @halmos in #228 (comment)

[crepererum]

Context:

This happens if we don't run the official app to receive all the emails from the inbound queue. We could improve the error message or skip affected emails, but I don't plan to implement the crypto to deal with this case.