Improve logging on listener bind

[syndicut]

Python 3.4.2
Rocket 1.2.5

You can get various exception on bind, in my case it was some issue with ipv6 bind and it was hard to debug without exception message:

ERROR:Rocket.Errors.Port8000:Socket ::1:8000 in use by other process and it won't share.
Traceback (most recent call last):
  File "/app/venv/lib/python3.4/site-packages/rocket/listener.py", line 85, in __init__
    listener.bind((self.addr, self.port))
socket.gaierror: [Errno -9] Address family for hostname not supported

So I suggest this little patch:
--- rocket/listener.py.old 2017-07-13 08:42:54.570297550 +0000
+++ rocket/listener.py 2017-07-13 08:37:40.115524793 +0000
@@ -85,7 +85,7 @@
             listener.bind((self.addr, self.port))
         except:
             msg = "Socket %s:%i in use by other process and it won't share."
- self.err_log.error(msg % (self.addr, self.port))
+ self.err_log.exception(msg % (self.addr, self.port))
         else:
             # We want socket operations to timeout periodically so we can
             # check if the server is shutting down