issue#9 bugfix: Go to definition opens blank tab when definition doesn't exist

+ added an `atom-sharper:error` event, currently is handled with dumb logging to console
+ raises error message "Can't navigate to '{{navigateToWord}}'" when try to go to definition and none exists
    + sorts out bug where this would previous open a new blank tab
    + we might want to refactor this out a bit, not sure if should all be in main atom-sharper.coffee?