child: Don't download child result fetch script if present

If the child pipeline test result fetcher script already exists in a
repo don't fetch it from this repo but use it instead to allow people to
customize the way the results are presented.
Don't use the failure block in the output xml for test errors, use the
error block instead and use the failure block to failures only.

Signed-off-by: Drew Reed <Drew.Reed@arm.com>