Interminable #inspect in Context

https://github.com/gregbell/arbre/blob/master/lib/arbre/context.rb#L90

is generating interminable (infinite?) inspect output that pushes the server's cpu to 100% enough time that I end up force killing it, I suggest implementing a custom `#inspect` or removing it from the error message.

(third option is a custom error class with an attributes that will store the context for inspection of an eventual `rescue` down the line)