Do not crash on kernel shutdow if json file is missing

[fperez]

I accidentally deleted my json connection file, but on shutdown IPython gave a crash message:

[NotebookApp] Shutting down kernels
Traceback (most recent call last):
  File "/home/fperez/usr/bin/ipython", line 7, in <module>
    launch_new_instance()
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/frontend/terminal/ipapp.py", line 389, in launch_new_instance
    app.start()
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/frontend/terminal/ipapp.py", line 359, in start
    return self.subapp.start()
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/frontend/html/notebook/notebookapp.py", line 581, in start
    self.cleanup_kernels()
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/frontend/html/notebook/notebookapp.py", line 544, in cleanup_kernels
    km.shutdown_kernel(kid)
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/frontend/html/notebook/kernelmanager.py", line 285, in shutdown_kernel
    super(MappingKernelManager, self).shutdown_kernel(kernel_id)
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/frontend/html/notebook/kernelmanager.py", line 103, in shutdown_kernel
    self.get_kernel(kernel_id).shutdown_kernel()
  File "/home/fperez/usr/lib/python2.7/site-packages/IPython/zmq/kernelmanager.py", line 839, in shutdown_kernel
    os.remove(self.connection_file)
OSError: [Errno 2] No such file or directory: '/home/fperez/.ipython/profile_default/security/kernel-570bd1e9-c2c0-4236-b3f5-c2cbacc5713b.json'

We should protect that os.remove call with a try/except and just print a warning if it's not there, but not crash.

[minrk]

I believe this is fixed in master now.