riseupvpn: future improvements

[bassosimone]

We originally used this issue to specify the following improvements:

• specific error for API failure (the endpoint is misconfigured?)
• specific error if JSON does not parse (ditto)

In reality, in 2023-10 I used an issue more generally as the reference for improving riseupvpn.

See #1432 (comment) for a discussion of the original objectives and what we have chosen to do instead, including references to follow up issues.

[bassosimone]

Of the two improvements mentioned above:

• specific error for API failure (the endpoint is misconfigured?)

The former has not been implemented, because we think we want to submit anyway. The crux of the matter has been to stop classifying the measurement inside the probe (ooni/probe-cli#1360) and the next logical step is to introduce more lax measurements classification in the backend (ooni/backend#745).

• specific error if JSON does not parse (ditto)

Because in ooni/probe-cli#1363 we said we wanted to use richer input (#2559) to serve targets to probes, this seems unnecessary, since this work would be done on the backend side.

[bassosimone]

Since I used this issue as a container for "riseupvpn improvements" and we have improvements and we have a more clear set of follow-up issues, I feel okay closing this issue now.