zephyr_mirror: Don't leak zephyr_mirror processes on send_humbug failures.

(imported from commit be005018672369683d9e3bdbf144f96f58af6b6c)

bots/zephyr_mirror_backend.py

@@ -340,10 +340,14 @@ def process_notice(notice, log):
 
     if os.fork() == 0:
         # Actually send the message in a child process, to avoid blocking.
+        try:
             res = send_humbug(zeph)
             if res.get("result") != "success":
                 logger.error("Error relaying zephyr:\n%s\n%s" % (zeph, res))
-        sys.exit(0)
+        except Exception:
+            logging.exception("Error relaying zephyr:")
+        finally:
+            os._exit(0)
 
 def decode_unicode_byte_strings(zeph):
     for field in zeph.keys():