bak_print_cmd fails when cfg['bak_open_exec'] missing

[ChanceNCounter]

Needs a try/except or a sanity check or something

[rexdivakar]

I can aslo look into this !

[ChanceNCounter]

It occurs to me that bak depends on an XDG-compliant, Unix-like system, which should always have diff. It's probably reasonable to just change the default config value to diff.

[ChanceNCounter]

Clearer: bak doesn't demand XDG compliance, but it falls back on the XDG spec when the XDG environment variables are absent.

Point is, not-Windowsness means diff should "always" exist.