Maintenance/operator error message (#659)

* update operator error message

* fix error message

Author: cbellot000

ansys/dpf/core/dpf_operator.py

@@ -108,7 +108,9 @@ def __init__(self, name, config=None, server=None):
             self._internal_obj = self._api.operator_new(self.name)
 
         if self._internal_obj is None:
-            raise KeyError(f"The operator {self.name} doesn't exist in the registry")
+            raise KeyError(f"The operator {self.name} doesn't exist in the registry. "
+                           f"Check its spelling in the documentation or verify its availability "
+                           f"in your Context (Entry/Premium) and in your loaded plugins.")
 
         self._spec = Specification(operator_name=self.name, server=self._server)
         # add dynamic inputs