Improve upgrade message when installed to a too small medium
In WhisperBack f6f1083a6fec826caac762d8f7664ae6 a user installed the .img on a 4 GB USB drive, i.e. less than the minimum, and thought it was a bug that they could not install automatic updates. We could fix this by improving the error message: currently it says something like "X is free but we need Y" (and Y > X), but I'm thinking we instead/also could check the size of the system partition and let the user know that they have installed Tails to a too small medium for upgrades to work and might want to re-install to a larger medium.