fix: ensure experiments return nil when we want to submit (#654)

Since https://github.com/ooni/probe-cli/pull/527, if an experiment
returns an error, the corresponding measurement is not submitted since
the semantics of returning an error is that something fundamental
went wrong (e.g., we could not parse the input URL).

This diff ensures that all experiments only return and error when
something fundamental was wrong and return nil otherwise.

Reference issue: https://github.com/ooni/probe/issues/1808.

internal/model/experiment.go

@@ -105,7 +105,7 @@ type ExperimentMeasurer interface {
 	// return an error in case the experiment could not run (e.g.,
 	// a required input is missing). Otherwise, the code should just
 	// set the relevant OONI error inside of the measurement and
-	// return nil. This is important because the caller may not submit
+	// return nil. This is important because the caller WILL NOT submit
 	// the measurement if this method returns an error.
 	Run(
 		ctx context.Context, sess ExperimentSession,