Return 401 if no token

[clausmichele]

Solves #82

Previously if an user was not authenticated and was requesting /jobs an error was produced and no proper json error message returned.

[pierocampa]

Ok, then we should not catch null pointers (a runtime non-checked exception), but rather avoid them at the root.
So just leave the generic Exception for now, and handle the case where the returned token is null. Then we're done! ; )