[tune] Improve error message when Ray crashes (#3795)

python/ray/tune/ray_trial_executor.py

@@ -9,6 +9,7 @@ import time
 import traceback
 
 import ray
+from ray.tune.error import TuneError
 from ray.tune.logger import NoopLogger
 from ray.tune.trial import Trial, Resources, Checkpoint
 from ray.tune.trial_executor import TrialExecutor
@@ -270,6 +271,11 @@ class RayTrialExecutor(TrialExecutor):
                 logger.warning("Cluster resources not detected. Retrying...")
                 time.sleep(0.5)
 
+        if not resources or "CPU" not in resources:
+            raise TuneError("Cluster resources cannot be detected. "
+                            "You can resume this experiment by passing in "
+                            "`resume=True` to `run_experiments`.")
+
         resources = resources.copy()
         num_cpus = resources.pop("CPU")
         num_gpus = resources.pop("GPU")