Handle errors in GCM responses

Delete devices from the database if the appropriate errors are returned.
If "error" is different than "Unavailable", delete the device.
    
This is according to:
http://developer.android.com/google/gcm/http.html

It's a style decision you have to make and set for the project. We use the regs_id short version and some other short versions around the code, and sometimes we use a full name (e.g collapse_key).

Please if you disagree with some of the style choices, feel free to correct it. I can (and just have) change this one, but this can be a never-ending game of changing names. :)