Update error message to state that exec_inspect is not supported

Author: zubron

docker/client.py

@@ -307,7 +307,9 @@ def exec_create(self, container, cmd, stdout=True, stderr=True, tty=False,
 
     def exec_inspect(self, exec_id):
         if utils.compare_version('1.16', self._version) < 0:
-            raise errors.InvalidVersion('Exec is not supported in API < 1.16')
+            raise errors.InvalidVersion(
+                'exec_inspect is not supported in API < 1.16'
+            )
         if isinstance(exec_id, dict):
             exec_id = exec_id.get('Id')
         res = self._get(self._url("/exec/{0}/json".format(exec_id)))