Handling error conditions on a web page for good user experience

For your website what are the best way to handle error conditions for effective end user experience ? Here are some scenarios for error conditions and I assume the answer is different for different scenarios:

1. When the whole page fails ?
2. When few modules on the page fails ?
3. When one free module on the page fails ?
4. When one paid module on the page fails ?

module is a section within your page

free module is a module that is available to all users

paid module is a module that is available to only paid members/premium users.