Refactor error/exception pages into an "Error handling" page

[Girgias]

This renames traditional errors to diagnostic errors Attempts to unify Throwable Errors with Exceptions Move throw, try, catch, and finally control structures to the control structure section

Feedback on this is very welcome and I'm open to suggestion, as some of the text still needs to be updated/improved. This is partly based on how Rust talks about error handling.

Some ideas I have also been considering:

• Split the two section in error-handling.xml into two files in a new subfolder error-handling/

A to-do list of sort:

• [x] Trim exception page
• [x] Maybe move extending exception page/trim it?
• [x] More examples in each specific section.
• [x] Create a new page in the control structure section for throw

Memo of things to do when this lands:

• Redirect previous pages to the new ones