Add a block for search results

This is a very useful module! I've had a few cases where we want to use the Search 404 functionality, but also need to include other content on the 404 page. Most recently this also included having the search results in the middle of some other content. To achieve this, I created a block for the Search 404 module that provides the search results.

To keep the block the same as the search results page, I refactored two small parts of the existing module:

1. Part of search404_page() was extracted into a new function search404_results() that is now used by the page and the block.
2. Another part of search404_page() was extracted into another function, search404_get_default_search(), for use in the page and the block.

I hope this will make a useful addition to this module. Thanks!