Add a hint to an error message about missing token

[HeeL]

I wanted to deploy my app with now and got the error message that says:
> Error! The content of "~/.now/auth.json" is invalid. No token property found inside

As its mentioning such words as auth & token, I guess its not too difficult to conclude that you have to go through some authorization process. Just to make it easier to understand that it's the case and also to give a hint how to do this, without looking into documentation, I think its a good idea to add a hint about this. Its just couple of words that clarify what is going on and what needs to be done ✌️

[codecov-io]

Codecov Report

Merging #1877 into canary will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           canary   #1877   +/-   ##
======================================
  Coverage    6.34%   6.34%           
======================================
  Files          93      93           
  Lines        4048    4048           
======================================
  Hits          257     257           
  Misses       3791    3791

Impacted Files	Coverage Δ
src/index.js	0% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5999ec7...56b760f. Read the comment docs.