TBD Quantitative indicators of data quality

### Context
We are already discovering flaws with some of the project data e.g. where the URL for the project's repository doesn't map to a current repo. (See https://github.com/commercetest/nlnet/issues/3 for various examples of the issues.)

Currently we skip many of these entries since we cannot discern which repo to query for tests. It may be useful to track and report these flaws e.g. to the team at NLnet with the aim of improving the upstream data about the projects.

The reporting might as well be in RDF format to facilitate automated consumption of the information. Therefore the results need to be in three column format AFAIK. TBD what information we can usefully provide and how to encode the results e.g. when would a binary result be acceptable (e.g. `valid|invalid`) vs. more nuanced results (e.g. `accessed_ok|unrecognised_as_a_code_repo|HTTP Error{nnn} received`)?

Start by reviewing issue #3 and the current handling of entries whose repository URLs cannot be mapped to a current repository. Determine which data-quality flaws can be observed and what RDF three-column results should report, including whether statuses should be binary or nuanced. The issue is complete when the indicators and output encoding are agreed and implementable.

Written by the indexing model from the issue text.