module I18n
  class MyExceptionHandler < ExceptionHandler
    def call(exception, locale, key, options)
      return 'ASD'
    end
  end
end

I18n.exception_handler = I18n::MyExceptionHandler.new


# vagrant@precise32:/vagrant$ rails c
# Loading development environment (Rails 4.1.0)
# 2.1.1 :001 > I18n::config.exception_handler
#  => #<I18n::MyExceptionHandler:0x891aad4>
# 2.1.1 :002 > I18n.exception_handler
#  => #<I18n::MyExceptionHandler:0x891aad4>
# 2.1.1 :003 >