Capture and return errors when singularity build fails

[prioux]

When a task performs a singularity build, if it fails it only loggs that it failed but there isn't much more information about that. I had a failure on a cluster simply because /usr/sbin wasn't in my PATH (and it's needed to run mksquashfs). Finding out required me to start a console on the bourreau, enter the task object, and invoke load_singularity_image_from_repo() manually to watch the traces.

A simple debugging fix would be to take the returned captured outputs of the tool_config_system() and store them in the task's work directory, for the admin to investigate.

[prioux]

Fixed in #832