Catch potential exceptions in the Stem event handler

The Stem Tor control port library starts an event notifier thread and
makes callbacks to the OnionBalance daemon with new events. If an
unhandled exception occurs in an event callback the event notifier
thread will crash. This will prevent OnionBalance getting new descriptor
updates and eventual the onion service will go offline.

This commit adds a `try: except` block around both event callback
functions which should gracefully handle unexpected errors in the
event notifier thread.

onionbalance/eventhandler.py

@@ -28,7 +28,12 @@ def new_status(status_event):
         if status_event.status_type == stem.StatusType.GENERAL:
             if status_event.action == "CONSENSUS_ARRIVED":
                 # Update the local view of the consensus in OnionBalance
-                consensus.refresh_consensus()
+                try:
+                    consensus.refresh_consensus()
+                except Exception:
+                    logger.exception("An unexpected exception occured in the "
+                                     "when processing the consensus update "
+                                     "callback.")
 
     @staticmethod
     def new_desc(desc_event):
@@ -59,7 +64,11 @@ def new_desc_content(desc_content_event):
             return None
 
         # Send content to callback function which will process the descriptor
-        descriptor.descriptor_received(descriptor_text)
+        try:
+            descriptor.descriptor_received(descriptor_text)
+        except Exception:
+            logger.exception("An unexpected exception occured in the "
+                             "new descriptor callback.")
 
         return None