Updated error message when getToken() returns "invalid_grant"
Updated error message when getToken() returns "invalid_grant"

Server time being out of sync has popped up a few times on Stack
Overflow
(http://stackoverflow.com/questions/10576386/invalid-grant-trying-to-get
-oauth-token-from-google and
http://stackoverflow.com/questions/41712965/how-to-connect-to-firebase-b
y-firebase-admin-against-a-proxy) and is probably just as common as the
case we already explain how to fix.

Change-Id: I9ba96a3c06106b7edcd75cff00e5f9e6de520cef

firebase/firebase-admin-node,firebase/firebase-admin-node