Change ErrNoKeyFound message
Change ErrNoKeyFound message

For #438. I was just going to change it to "wrong password" but then I saw that it might actually be the case that no key could be found, so I changed it to what I did. Let me know if you'd like something different!
ar-jan/restic,kurin/restic,byo/restic,stuertz/restic,mappu/restic,restic/restic,fawick/restic,byo/restic,fawick/restic,restic/restic,middelink/restic,middelink/restic,ckemper67/restic,ar-jan/restic,jayme-github/restic,arithmetric/restic,stuertz/restic,kurin/restic,jayme-github/restic