Available disk space check

For robustness as well as improved user experience we should consider adding proactive disk space availability checks in several spots[1] before proceeding with material disk using steps:

• at start (a general check for the prevailing then system requirement/recommended amount of available space before doing an update/upgrade: e.g. 2 x 1GiB or whatever)
• at start (a general check, based on the user's current installation, to make sure they have at least 2x their current installations space usage, excluding datadirectory, available)
• createBackup() (e.g. enough space before proceeding with the backup)
• downloadUpdate() (e.g. enough space for the download)
• extractDownload() (e.g. enough space for the extraction)

Possible checks might include one or more of:

• warn before proceeding with the step if disk space is lower than a baseline absolute number we consider necessarily to complete that step typically
• use real numbers where it's an option (e.g. we may be able to know the download size before we download, we may be able to calculate the space needed to perform the pre-update backup)

Things to consider:

• one check at the beginning isn't necessarily enough since systems are dynamic, but in some cases it's probably "good enough"

Benefits:

• better user experience through enhanced transparency, preventing execution of steps that are likely to fail, and elimination of partial updates
• lowered false bug reports (i.e. don't blame the Updater for local environment limitations)

[1] Or just one spot, at least to start, if we can cover something like 80% of the situations without needing new checks everywhere