avoid KeyError when unregistering an engine twice

the new KernelNanny can mean that engine unregistration is requested twice

avoid KeyError and skip when unregistering an engine that has already been removed

Author: minrk

ipyparallel/controller/hub.py

@@ -913,14 +913,17 @@ def unregister_engine(self, ident, msg):
             eid = msg['content']['id']
         except:
             self.log.error(
-                "registration::bad engine id for unregistration: %r",
-                ident,
-                exc_info=True,
+                f"registration::bad request for engine for unregistration: {msg['content']}",
             )
             return
-        self.log.info("registration::unregister_engine(%r)", eid)
-
+        if eid not in self.engines:
+            self.log.info(
+                f"registration::unregister_engine({eid}) already unregistered"
+            )
+            return
+        self.log.info(f"registration::unregister_engine({eid})")
         ec = self.engines[eid]
+
         content = dict(id=eid, uuid=ec.uuid)
 
         # stop the heartbeats