Handle second instance of WebUI nicely

[Cas]

If you run a second instance of deluge-web it fails with an exception log, we should handle this in a nicer manner.

20:26:26.566 [ERROR   ][deluge.ui.web.web             :107 ] Couldn't listen on 0.0.0.0:8112: [Errno 98] Address already in use.
Traceback (most recent call last):
  File "~/deluge/deluge/ui/web/web.py", line 105, in run
    self.server.start()
  File "~/deluge/deluge/ui/web/server.py", line 617, in start
    self.start_normal()
  File "~/deluge/deluge/ui/web/server.py", line 625, in start_normal
    self.socket = reactor.listenTCP(self.port, self.site, interface=self.interface)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/posixbase.py", line 478, in listenTCP
    p.startListening()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/tcp.py", line 984, in startListening
    raise CannotListenError(self.interface, self.port, le)
CannotListenError: Couldn't listen on 0.0.0.0:8112: [Errno 98] Address already in use.
Traceback (most recent call last):
  File "~/delugeDev/deluge-web", line 9, in <module>
    load_entry_point('deluge==1.4.0.dev629', 'console_scripts', 'deluge-web')()
  File "~/deluge/deluge/ui/web/__init__.py", line 5, in start
    Web().start()
  File "~/deluge/deluge/ui/web/web.py", line 109, in start
    run_profiled(run, output_file=self.options.profile, do_profile=self.options.profile)
  File "~/deluge/deluge/common.py", line 1038, in run_profiled
    return func(*args)
  File "~/deluge/deluge/ui/web/web.py", line 105, in run
    self.server.start()
  File "~/deluge/deluge/ui/web/server.py", line 617, in start
    self.start_normal()
  File "~/deluge/deluge/ui/web/server.py", line 625, in start_normal
    self.socket = reactor.listenTCP(self.port, self.site, interface=self.interface)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/posixbase.py", line 478, in listenTCP
    p.startListening()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/tcp.py", line 984, in startListening
    raise CannotListenError(self.interface, self.port, le)
twisted.internet.error.CannotListenError: Couldn't listen on 0.0.0.0:8112: [Errno 98] Address already in use.

[Cas]

Fixed: [7b523af05bc8e]

[Cas]

Milestone renamed