Catch exceptions when shutting down communication port.

[kevin-bates]

After running tests over the weekend, it looks like a recent change for PR #279 should catch exceptions when attempting to shutdown the connection. This is leading to issues surrounding the culling of kernels.

[E 2018-03-10 05:13:28.718 EnterpriseGatewayApp] The following exception was encountered while checking the idle duration of kernel 4d172f2b-1afa-4edb-a7df-dd588252bce0: [Errno 107] Transport endpoint is not connected
    Traceback (most recent call last):
      File "/opt/anaconda2/lib/python2.7/site-packages/notebook/services/kernels/kernelmanager.py", line 414, in cull_kernels
        self.cull_kernel_if_idle(kernel_id)
      File "/opt/anaconda2/lib/python2.7/site-packages/notebook/services/kernels/kernelmanager.py", line 435, in cull_kernel_if_idle
        self.shutdown_kernel(kernel_id)
      File "/opt/anaconda2/lib/python2.7/site-packages/notebook/services/kernels/kernelmanager.py", line 267, in shutdown_kernel
        return super(MappingKernelManager, self).shutdown_kernel(kernel_id, now=now)
      File "/opt/anaconda2/lib/python2.7/site-packages/jupyter_client/multikernelmanager.py", line 33, in wrapped
        r = method(*args, **kwargs)
      File "/opt/anaconda2/lib/python2.7/site-packages/jupyter_client/manager.py", line 314, in shutdown_kernel
        self.request_shutdown(restart=restart)
      File "/opt/anaconda2/lib/python2.7/site-packages/enterprise_gateway/services/kernels/remotemanager.py", line 170, in request_shutdown
        self.process_proxy.shutdown_listener()
      File "/opt/anaconda2/lib/python2.7/site-packages/enterprise_gateway/services/processproxies/processproxy.py", line 779, in shutdown_listener
        sock.shutdown(SHUT_WR)
      File "/opt/anaconda2/lib/python2.7/socket.py", line 228, in meth
        return getattr(self._sock,name)(*args)
    error: [Errno 107] Transport endpoint is not connected