Warn instead of raising on invalid action

gym/core.py

@@ -1,4 +1,6 @@
 import logging
+logger = logging.getLogger(__name__)
+
 import numpy as np
 
 from gym import error, monitoring
@@ -79,7 +81,8 @@ def step(self, action):
         info (dict): contains auxiliary diagnostic information (helpful for debugging, and sometimes learning)
         """
         if not self.action_space.contains(action):
-            raise error.InvalidAction('Invalid action selected: {}'.format(action))
+            hint = self.action_space.sample()
+            logger.warn("Action '{}' is not contained within action space '{}'. HINT: Try using a value like '{}' instead.".format(action, self.action_space, hint))
 
         self.monitor._before_step(action)
         observation, reward, done, info = self._step(action)