Return delimiter when file was not found

Hi, we want to use the http-concat module to batch get multiple files (not only css/javascript) and we're currently using a custom `concat_delimiter` to be able to split on that and get the body for each file individually.

However, when requesting a file that doesn't exist (we're using `concat_ignore_file_error on`) the file gets ignored but no delimiter gets added. At that point our UI has no idea which file wasn't found.
Would it be feasable to add an empty body (even better: have something configurable like `concat_not_found_body`) _and_ the delimiter? That way, if you request 4 files, you always get 3 delimiters. (and thus 4 separate bodies)

Start by tracing the http-concat module's handling of concat_ignore_file_error and concat_delimiter. Verify how missing files are represented, then define done as preserving the delimiter for each requested file and, if supported, emitting the configured concat_not_found_body; test a request containing both existing and missing files.

Written by the indexing model from the issue text.