send backup: fail quickly on insufficient storage

[r10s]

receiving a large backup can take several dozen minutes - if, at 90%, the device is out of storage, this is very frustrating - not only that the backup did not succeed, also much time was wasted.

this issue is especially true for low-budget devices, which have poor transfer rates due to poor wi-fi hardware - and often little storage space as well. plus, there may be an overall poor network, bad router, far away from router ...

my suggestion is that dc_receive_backup() returns as soon as possible if it is clear that there is insufficient storage, with a nice error set (so no UI adaptions are needed).

my first idea was to add the storage needed to the qr code, however, @dignifiedquire pointed out in the Dev chat that additional information in the qr code are not needed, iroh already knows the size, so it’s a matter of figuring out what to do with that info.

while being a huge UX improvement, much more than "nice to have", this issue is still an optimisation, so i would not see that as absolutely necessary for a first release. depends a bit on how hard it is to do :)