(maint) Clarify the error message when a relationship can't be established
The following error message:
Error: Could not retrieve catalog from remote server: Could not intern from text/pson: Could not intern from data: Could not find relationship target "File[]"
Will get additional information:
Error: Could not retrieve catalog from remote server: Could not intern from text/pson: Could not intern from data: Could not find relationship target "File[]" for Apache::Vhost[example.com]
So the error is easier to debug.