juliaup destroyed config file when running out of disk quota

[eschnett]

I ran out of disk quota while juliaup was running:

$ juliaup update
Checking for new Julia versions
Error: Failed to update versions db.

Caused by:
    0: Failed to save configuration file.
    1: Failed to write config data to disc.
    2: Quota exceeded (os error 122)

After this, the file .julia/juliaup/juliaup.json did exist but had length zero:

$ juliaup status
Error: `status` command failed to load configuration file.

Caused by:
    0: Failed to parse configuration file '"/home/eschnetter/.julia/juliaup/juliaup.json"' for reading.
    1: EOF while parsing a value at line 1 column 0

I ended up deleting my .julia/juliaup directory to correct this issue (after ensuring I have quota left to write more files).

I think juliaup may be using an unsafe way to update this config file. It might be better to update the config by writing to a new file, and then renaming the new to the old file. This would allow reverting to a good state even when further disk writes are impossible.