Handle 'deletable' cell metadata
Add tests for undeletable cells
Make cell be undeletable ONLY when metadata is explicitly false
Fix tests
Properly mock winreg functions for test

Fixes a lot of test failures on Python 3.4 on Windows, caused by
importlib trying to use our mocked out winreg functions when
reload(path) was called.
Handle NoSuchKernel errors more gracefully
Show the user a different notification
Report the exact error that occurred
Remove 'we' from message
Better user experience when kernel isn't found
Use 501 error code instead of 400
Return a proper JSON object
Always show the modal dialog, and have a fallback generic message
Don't pop up modal dialog
More descriptive short message
s/long thing/set_state
s/_handle_receive_state/set_state in tests too
s/apply_update/set_state
Merge pull request #6581 from ipython/mock-winreg-tests

Properly mock winreg functions for test
Interpret another error as missing symlink support on Windows

This occurs if the system and Python support symlinks, but the user does
not have permission to create them.