Improve logging when locust master port is busy.

[cyberw]

Avoid issues like https://stackoverflow.com/questions/62769727/facing-error-address-already-in-use-while-running-locust

Instead of printing:

[2020-07-07 12:26:25,897] sonali-Latitude-3490/ERROR/stderr: Traceback (most recent call last):
[2020-07-07 12:26:25,897] sonali-Latitude-3490/ERROR/stderr: File "/home/sonali/.local/bin/locust", line 8, in <module>
[2020-07-07 12:26:25,897] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,897] sonali-Latitude-3490/ERROR/stderr: sys.exit(main())
[2020-07-07 12:26:25,897] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,897] sonali-Latitude-3490/ERROR/stderr: File "/home/sonali/.local/lib/python3.6/site-packages/locust/main.py", line 503, in main
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: runners.locust_runner = MasterLocustRunner(locust_classes, options)
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: File "/home/sonali/.local/lib/python3.6/site-packages/locust/runners.py", line 320, in __init__
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: self.server = rpc.Server(self.master_bind_host, self.master_bind_port)
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: File "/home/sonali/.local/lib/python3.6/site-packages/locust/rpc/zmqrpc.py", line 41, in __init__
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: self.socket.bind("tcp://%s:%i" % (host, port))
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: File "zmq/backend/cython/socket.pyx", line 550, in zmq.backend.cython.socket.Socket.bind
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: File "zmq/backend/cython/checkrc.pxd", line 26, in zmq.backend.cython.checkrc._check_rc
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: zmq.error
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: .
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: ZMQError
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: :
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: Address already in use
[2020-07-07 12:26:25,898] sonali-Latitude-3490/ERROR/stderr: 

It will print something like:

[2020-07-07 10:19:28,906] lafp-mac-JG5J.int.svenskaspel.se/ERROR/locust.runners: The Locust master port (5557) was busy. Close any applications using that port - perhaps an old instance of Locust is still running? (Socket bind failure: Address already in use)

[codecov]

Codecov Report

Merging #1471 into master will increase coverage by 0.35%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1471      +/-   ##
==========================================
+ Coverage   81.11%   81.47%   +0.35%     
==========================================
  Files          27       27              
  Lines        2378     2386       +8     
  Branches      365      366       +1     
==========================================
+ Hits         1929     1944      +15     
+ Misses        357      351       -6     
+ Partials       92       91       -1     

Impacted Files	Coverage Δ
locust/runners.py	80.61% <77.77%> (+0.32%)	⬆️
locust/stats.py	89.01% <0.00%> (+0.22%)	⬆️
locust/user/task.py	96.21% <0.00%> (+0.54%)	⬆️
locust/clients.py	95.09% <0.00%> (+4.90%)	⬆️

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