Show an error instead of a stacktrace when a file doesn't exist.

Show a nice error instead of a stacktrace. Makes it easier to see what is actually wrong.
Note: if this is something you guys don't want, just cancel the PR :)

Note: I did not check if it is a directory because I might want to implement the closed #134 if you guys want me to? I will add a question to that issue right after this one :)