securejoin: observing device posts error when receiving `vg-request-with-auth`

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See comment in https://github.com/deltachat/deltachat-core-rust/pull/5930#issuecomment-2319934675

1. Alice sets up two devices.
2. Alice puts the second device offline.
3. Alice creates unpromoted group on the first device.
4. Alice creates securejoin QR code for the group.
5. Bob scans the QR code.
6. Alice's first device runs securejoin protocol to the end and adds Bob to the group with vg-member-added message.
7. Alice puts the second device online.

By the time Alice's second device sees vg-request-with-auth message, it posts error messages "Cannot establish guaranteed end-to-end encryption with ...". Probably happens here: https://github.com/deltachat/deltachat-core-rust/blob/c7c3b9ca9003f0abc5c5e86af363e1c628fef2b6/src/securejoin.rs#L418-L427

I think Alice should not post error messages if securejoin fails. It is Bob who scanned the QR code and should care about securejoin finishing successfully. If securejoin does not succeed, Alice should not see anything at all.