Exit 125 when ~/.fnm/node-versions does not exist

[taylor1791]

If you run fnm ls before installing any versions, you get an error since ~/.fnm/node-versions has not been created.

$ fnm ls
fnm: internal error, uncaught exception:
     (Failure "/Users/xxx/.fnm/node-versions: No such file or directory")
     Raised at file "executable/ListLocal.re", line 6, characters 6-554
     Re-raised at file "src/core/lwt.ml", line 2987, characters 20-29
     Called from file "src/unix/lwt_main.ml", line 26, characters 8-18
     Called from file "cmdliner_term.ml", line 25, characters 19-24
     Called from file "cmdliner.ml", line 116, characters 32-39
$ echo $?
125

[Schniz]

Hey! thanks for the bug report and sorry about that - you're right - we can actually print "nothing is here" or just exit