SPARK-532 Log something when no resources have been offered in the cluster

This solves a long-standing usability issue, that jobs seem to just "hang" when they are submitted to a cluster without enough resources.

This patch adds two warning messages that address two variants of this problem:

(a) A job is submitted to the cluster scheduler, but there aren't any slaves registered yet with the scheduler. Therefore, no offers are made.
(b) Slaves are registered with the Standalone scheduler, but none have enough memory to launch executors on.

This is a preliminary patch which handles the case where these errors happen on cluster startup. Some types of "starvation" are not covered, such as a job arriving in an already full cluster that needs to wait until other jobs finished (it's much more subjective what to log and when in that situation).