Raise an error when path is specified with :moved_from or :moved_to

[kenhys]

If same path is specified with :moved_from or :moved_to
additionally, :modify is not fired expectedly.

  require "rb-inotify"
  notifier = INotify::Notifier.new
  notifier.watch("test.log", :modify) do |event|
    puts "#{event} is modified"
  end
  notifier.watch("test.log", :moved_from) do |event|
    puts "#{event} is moved_from"
  end

Above sample code does not work as expected.
So, raise an error immediately.

[coveralls]

Coverage increased (+0.2%) to 80.556% when pulling bf2fc88 on kenhys:error-moved-to-file into 010f140 on guard:master.