Make the "not enough bots" error message more explicit

Bug: 878527
Change-Id: I2e527d39337232a657c34aa51c021d82b5f8a02d
Reviewed-on: https://chromium-review.googlesource.com/c/1347054Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Brad Hall <bradhall@google.com>
Cr-Commit-Position: refs/heads/master@{#610168}

testing/trigger_scripts/perf_device_trigger.py

@@ -129,7 +129,9 @@ class PerfDeviceTriggerer(base_test_triggerer.BaseTestTriggerer):
         self._print_device_affinity_info({}, {},
           self._eligible_bots_by_ids, args.shards)
       raise ValueError('Not enough available machines exist in in swarming'
-                       'pool. Contact labs to rack in more hardware')
+                       'pool.  Shards requested (%d) exceeds available bots '
+                       '(%d).' % (
+                           args.shards, len(self._eligible_bots_by_ids)))
 
     shard_to_bot_assignment_map = {}
     unallocated_bots_by_ids = copy.deepcopy(self._eligible_bots_by_ids)