Update Nvim error handling

Now errors sent from Nvim are [code, message] arrays, this adapts the error
handling code to account for that.
Ignore SIGINT when talking to nvim via stdio

This is necessary because the python host runs in the same session as nvim(and
receives SIGINT when the users presses ctrl+C)
Merge pull request #33 from tarruda/api-error-improvements

Update Nvim error handling