Add some code to facilitating the debugging of bad test data from a source

[cwant]

In particular, don't let the sources/show page crash with a 500.

Summary of changes

Before, if you are testing a source and you pull in some bad data that won't render, you are stuck and you can't see the page related to your source until you delete the file pointed by test_results_path.

This is particularly problematic if you don't have access to the back end. In production, the only clue you have is a message saying "500 Internal Server Error".

With this change, not only will the server not throw a 500, it will show some information on the #testing tab for the source that might help fix things (and will give you access to the show page for the source again.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.

[cwant]

By the way, if you would like a test ingestion that exhibits the problem, set up a CSV materials source with this URL:

https://gitlab.com/rzon/myawesometest/-/raw/66ee6c905472afc011107a726ddfd3fa670a59aa/scinet_training_material.csv

Do a test ingestion on the webpage, then the sources page crashes.