Delay DidYouMean until NotFoundError#message is called

[tompng]

expand_class is slow for long class name that has no document.

irb(main):001> driver = RDoc::RI::Driver.new
=> 
#<RDoc::RI::Driver:0x0000000105c03180
...
irb(main):002> measure
TIME is added.
=> nil
irb(main):003> driver.expand_class('MyFooBarApp::FooBarController::FooBar') rescue nil
processing time: 0.227079s

It's because did_you_mean is slow and always calculated before raising NotFoundError.

This pull request delays calculation of DidYouMean until NotFoundError#message is called.
Processing time reduces 0.227079s → 0.006436s

Background

This will fix ruby/irb#709

[MatheusRich]

Looks good to me. TY