flatpak: Introduce success/failure heuristics for app install
We want to pro-actively fail the install operations if we are sure that we are running low on disk-space. This saves the network bandwidth for user and reduce this cycle: online fetch -> detect low disk free space -> cleanup.
- For app install: We make sure that there is enough space for app's download size + min-free-space (flatpak defaults to 500MB right now).
We might be interested to do similar for app updates and autoupdates in following commits.