Make WDL errors and container dumps trace where files came from

[unito-bot]

When a WDL job fails or you debug it, it will print a bunch of file paths. toil debug-job --retrieveTaskDirectory in particular announces all the host-side files mounted into the container.

We should automatically go back from local paths to the WDL-visible variable names that allowed the task to get the virtualized filename that we devirtualized to get these files. So when the user is looking at random-file-name.txt they can also have the context that this was input_file_list[3] for the task or PrevTask.output3 for a workflow-level expression.

┆Issue is synchronized with this Jira Story ┆Issue Number: TOIL-1587