Some information on quotas when using the DataStore back end in Database abstraction with google apps script
The library reference is 



If you are using the free tier for DataStore, you’ll run out of quota at some point. Rate limited quotas are handled automatically by the Backing off on rate limiting capability used by the DataHandler, but there are some hard quota limits. You can find the most useful information on DataStore usage in your cloud console under the AppEngine section for your app.

Here’s what you’ll find there when you are out of quota. Datahandler will return an http error 402, which means RESOURCE EXHAUSTED, and you’ll see something like this in your Appengine console.

There isn’t a way round this if you are not willing to pay for DataStore usage. However it’s pretty easy when developing to make multiple projects, as described in Datastore driver and flip between them, once you’ve registered the credentials by changing the property name under which the credentials are stored. Below I’m using the ‘xliberationdatastoredev’ datastore.

See more like this in Database abstraction with google apps script and Datastore driver