After the workload fails due to OverloadedException, c-s returns a misleading error message

[michoecho]

Commit 0356275 added a special code path (with timeouts) for OverloadedException, but this path doesn't set any value for exceptionMessage. If all retries fail with OverloadedException then we exit the retry loop with success set to false but exceptionMessage set to null, which to later code means that validation failed (i.e. the result was incorrect) and cassandra-stress fails with Data returned was not validated, which is the error message for incorrect results.

The OverloadedException handler should be set exceptionMessage to some error text (e.g. "OverloadedException" or whatever) so that c-s fails with an accurate error message.