taskhandler suppress cancel error

Added an errback to handle errors within task handler's deferred instance.
Without this instances of deferred cause a traceback when a deferred is canceled without errback or callback having been called.
This traceback does not end execution, and ultimately would only show to main console.
Reference cancel: https://github.com/twisted/twisted/blob/trunk/src/twisted/internet/defer.py

All evennia unit tests pass.

evennia/scripts/taskhandler.py

@@ -6,6 +6,7 @@ from datetime import datetime, timedelta
 
 from twisted.internet import reactor
 from twisted.internet.task import deferLater
+from twisted.internet.defer import CancelledError as DefCancelledError
 from evennia.server.models import ServerConfig
 from evennia.utils.logger import log_err
 from evennia.utils.dbserialize import dbserialize, dbunserialize
@@ -13,6 +14,17 @@ from evennia.utils.dbserialize import dbserialize, dbunserialize
 TASK_HANDLER = None
 
 
+def handle_error(*args, **kwargs):
+    """
+    Handle errors withing deferred objects.
+    """
+    for arg in args:
+        # suppress cancel errors
+        if arg.type == DefCancelledError:
+            continue
+        raise arg
+
+
 class TaskHandler(object):
 
     """
@@ -179,7 +191,9 @@ class TaskHandler(object):
             deferLater(self.clock, timedelay, callback, *args, **kwargs)
             return task_id
 
-        return deferLater(self.clock, timedelay, callback, *args, **kwargs)
+        d = deferLater(self.clock, timedelay, callback, *args, **kwargs)
+        d.addErrback(handle_error)
+        return d
 
     def remove(self, task_id):
         """Remove a persistent task without executing it.