Simplify "notify" for error reporting

[kyrylo]

Error notifications are currently supported via airbrake.notify(obj) and airbrake.notify(error: obj). This is really confusing and makes internal implementation difficult to maintain. I believe we should deprecate airbrake.notify(obj) in favor of airbrake.notify({ error: obj }).

I will spend more time thinking about this. For now, I wrote this down so that I don't forget about it.