Log exception when bot airbag is activated

To facilitate investigating airbag-prevented PTGbot crashes,
log the exception stacktrace.

Change-Id: I6600abc1f07f16b739c62aeb632bd5bfd9a5e3e9

ptgbot/bot.py

@@ -54,7 +54,7 @@ def make_safe(func):
             func(*args, **kwargs)
         except Exception as e:
             msg = "Bot airbag activated: " + str(e)
-            args[0].log.error(msg)
+            args[0].log.error(msg, exc_info=True)
             args[0].send(args[0].channel, msg)
     return inner