Improve error messages of ServerProcessesManager for the imaging uploader

[nicolasbrossard]

The exit codes of the imaging pipeline have been documented and centralized in a file in the LORIS-MRI codebase. The ServerProcessesManager page could be improved in order to benefit from this, printing a description for the exit code along with the exit code itself. Architecture/design TBD.

Original Redmine ticket: https://redmine.cbrain.mcgill.ca/issues/13941

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nicolasbrossard]

Still stands as of 2024SEP17