Add check on links

[gfriloux]

rotz is missing a sanity check on links.

PEBKAC from your servitor here happened:
I have a file named config declared as :

links:
  condig:~/.gitconfig

As you can see, there is a typo in the name.
rotz then created a dead link.

i don't think rotz should create deadlinks, it should error, telling the file condig does not exists (and optionally, telling me i am stupid :) ).

PS: Thanks for the tool!

[volllly]

Hey @gfriloux, thanks alot for reporting :)
I've added checks and a nice error message in the just released v0.9.3