Surface DNS failures as unexpected results

[fortuna]

When the DNS server returns SERVFAIL, which is the predominant interference by CANTV in Venezuela, OONI says the test failed. Example: https://explorer.ooni.org/m/20231227230204.056784_VE_webconnectivity_f12c60cdf02a28ae

The errors surfaced in the DNS Queries are very helpful in diagnosing the problem, but it is extra work. That means I can't trust failed measurements and I have to dig manually deeper. The fact that MAT shows up as mostly gray (it could be showing "DNS: Servfail" instead), reduces its value, since I need to check the entries.

The measurement search, when you restrict to CANTV, is misleading, not showing the DNS interference, since failed measurements are omitted by default: https://explorer.ooni.org/search?since=2023-11-28&until=2023-12-29&failure=false&domain=psiphon.ca&probe_cc=VE&probe_asn=AS8048.

My proposed fix here is to treat all DNS measurements using the system resolver as valid, regardless of the error. At a minimum consider any error that indicates a server response as a valid measurement.