Credentials check before read

[samjett247]

This pull request addresses the enhancement in #111 . Basically, if a user follows the quickstart instructions out of order by running python quickstart.py before downloading credentials.json in step 1 of the API quickstart, the program throws an uncaught error - "cannot find 'credentials.json'". I added an error catch for this in sheets/quickstart that directs the user to follow Step 1. If this solution looks okay I can make the changes to each of the other quickstarts.