Added support for JSON errors, objects in error! and handling runtime exceptions within Grape

[dblock]

• The API has been extended to support three new options.
  • rescue_all_errors true # traps all exceptions within grape
  • error_format :json # returns errors in JSON format
  • default_error_status 200 # changes default status of raise-d errors from 403 to a custom value
• Error can raise error!(hash, code) and pass that through
• Error middleware has been extended to support these custom formatters.