EjrApiResponseError: improve error message

[bossie]

Context: I forgot to add a field to the EJR API so batch jobs broke; in particular, starting a batch job made OpenEO return this 500 error in response to a 400 from the EJR API:

openeo.rest.OpenEoApiError: [500] Internal: Server error: EjrApiResponseError('Error communicating with batch job system, consider trying again. Details: 400 'Bad Request' on PATCH \'https://jobregistry.vgt.vito.be/jobs/j-2507162224414dd5ae34b7bee23d713a\': {"statusCode":400,"message":["property results_metadata_uri should not exist"],"error":"Bad Request"}') (ref: r-250716222441465198d52a399903d41c)

The error message suggests to try again but this is obviously not going to work in this case. A better approach could be to adapt this response message (and status code) to the one that we get back from the EJR API.

[soxofaan]

A better approach could be to adapt this response message (and status code) to the one that we get back from the EJR API.

I'm not sure about that: the EJR is an implementation aspect of the openEO backend. It is not a concern to expose the end-user to.
I think it's perfectly right that the 400 error from the EJR request is exposed as a 500 to the end user. In this case, it's the openEO backend that made a "client error", but that's internal business, so it should be propagated to the end-user as "500 Internal". There is nothing the end user can do to fix anything, so they should not get a 4xx response

Regardless of the status code, there is of course still room to improve the message itself

[bossie]

Sure: adapt, not just propagate.