run-dev: Make address-in-use errors more obvious.
run-dev: Make address-in-use errors more obvious.

This seems to be a common enough pitfall to justify
a bit of extra handling.  Example output:

    $ ./tools/run-dev.py
    Clearing memcached ...
    Flushing memcached...
    OK
    Starting Zulip services on ports: web proxy: ...
    Note: only port 9991 is exposed to the host in a Vagrant environment.

    ERROR: You probably have another server running!!!

    Traceback (most recent call last):
      File "./tools/run-dev.py", line 421, in <module>
        app.listen(proxy_port, address=options.interface)
      File "/srv/zulip-py3-venv/lib/python3.5/...
        server.listen(port, address)
      File "/srv/zulip-py3-venv/lib/python3.5/...
        sockets = bind_sockets(port, address=address)
      File "/srv/zulip-py3-venv/lib/python3.5/...
        sock.bind(sockaddr)
    OSError: [Errno 98] Address already in use
    Terminated

synicalsyntax/zulip,rishig/zulip,tommyip/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,showell/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,andersk/zulip,timabbott/zulip,brainwane/zulip,synicalsyntax/zulip,kou/zulip,shubhamdhama/zulip,showell/zulip,synicalsyntax/zulip,andersk/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,eeshangarg/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,kou/zulip,shubhamdhama/zulip,rht/zulip,kou/zulip,brainwane/zulip,rishig/zulip,rishig/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,showell/zulip,rht/zulip,zulip/zulip,rishig/zulip,kou/zulip,timabbott/zulip,punchagan/zulip,timabbott/zulip,rht/zulip,brainwane/zulip,zulip/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,shubhamdhama/zulip,rishig/zulip,showell/zulip,shubhamdhama/zulip,hackerkid/zulip,rishig/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,tommyip/zulip,eeshangarg/zulip,zulip/zulip,timabbott/zulip,andersk/zulip,tommyip/zulip,hackerkid/zulip,timabbott/zulip,rht/zulip,rishig/zulip,synicalsyntax/zulip,zulip/zulip,rht/zulip,timabbott/zulip,showell/zulip,tommyip/zulip,shubhamdhama/zulip,tommyip/zulip,synicalsyntax/zulip,punchagan/zulip,tommyip/zulip,synicalsyntax/zulip,showell/zulip,tommyip/zulip