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/engine/experiment/tor/tor.go

@@ -174,7 +174,7 @@ func (m *Measurer) Run(
 ) error {
 	targets, err := m.gimmeTargets(ctx, sess)
 	if err != nil {
-		return err
+		return err // fail the measurement if we cannot get any target
 	}
 	registerExtensions(measurement)
 	m.measureTargets(ctx, sess, measurement, callbacks, targets)