Check free disk space on tmp volume before backup

[elmariofredo]

When you will try to backup huge dir, which will not fit into tmp volume tergum will fail without explanation. We should check if there is approximately enough space and return warning if tmp dir might be too small.

tergum/backup/source/dir/dir.go

Line 28 in 9b8a48a

outputFile, err := os.CreateTemp("", "tergum-tar-gz-")

As workaround you can set variable TMPDIR to point to volume with enough space.