All different kinds of errors our server sends are in compliance with Amazon S3 errors. They are now stored in one place and should be accessed from anywhere.

Changelog:
* add the file s3_errors.h that contains const objects for all errors that might occur (most of them are not checked yet)
* change struct error_info into a class and add a function to create the XML from the object's variables
* Use these errors in the request handlers
* Add an error object to the s3 authorisation that will always contain "Access Denied" or another error if applicable
* Update tests to test the code generation and error return values