redirects to main_app.root_url when folder id is invalid

[jiaola]

We found a problem a user tries to access a folder that doesn't exist or has been deleted. It throws an error: ActiveRecord::RecordNotFound in Blacklight::Folders::FoldersController#show

We'd like to catch this error, redirect to root_url with a helpful message. This fix does that.

This is a resubmit of the previous pull request #98

[cjcolvar]

Would it be better to go to folders index instead of app home?

[jcoyne]

Rails catches the RecordNotFound errors in production mode and shows a 404 page. https://github.com/rails/rails/blob/504e5e102b847019f08c4c2d1fa30444ed9a59a0/railties/test/application/middleware/exceptions_test.rb#L35-L46 So we probably don't want your PR to prevent that from happening.