failing `dc_receive_backup()` lacks last error set

[r10s]

while trying to transfer a large database (>4gb) from ios to android 1.35.2, i got the empty error that was reported initially by @hpk42 :

i meanwhile added some context to the alerts, so the error it looks like:

little better, but not good :)

the log, otoh, reports the real issue:

 W  deltachat-ffi/src/lib.rs:4239: Get backup failed: No space left on device (os error 28)

i think, we should report that with error! - then also last_error get set and the UI will show the real cause.

also, the transfer is left in a "half state" with lots of images etc. missing - i think, the account should be left unconfigured, deleted or so.

nb: the sender in dc_backup_provider_wait() also fails, this is nice - however, again, with unspecific error as well.

related: related to #4178

nb: we should consider to add the needed space to the qr code and not start, if it is clear it is to few space (getting that error only after 30 minutes is pretty bad ux) (this could be done completely in core at some later point by just leaving receive_backup fail with an error set)