Improve message for 'ResourceShaclEngineRejection'

[imsdu]

Fixes #4830

In the end, the problem was not related to a resource not found but to a static remote context (the shacl one provided by Nexus) required to generate the compacted representation of the validation report.
In the end, the problem happened because the import batch did not inject this static context so this PR is:

• Enriching the error message to make it clearer
• Giving it a 5xx status as it is a problem of the application

[shinyhappydan]

If the resource isn't found shouldn't we end up with a 404? Feels like we could split the error into two: a shacl engine error and a resource not found

[imsdu]

If the resource isn't found shouldn't we end up with a 404? Feels like we could split the error into two: a shacl engine error and a resource not found

Sorry, I should have made it clearer on what was the solution. I edited the description of this PR.