fix(measurex): use same keys of the OONI data format (#572)

This change should simplify the pipeline's job.

Reference issue: https://github.com/ooni/probe/issues/1817.

I previously dismissed this possibility, but now it seems clear it
is simpler to have a very tabular data format internally and to
convert such a format to OONI's data format when serializing.

The OONI data format is what the pipeline expects, but processing
is easier with a more linear/tabular format.

internal/measurex/failure.go

@@ -0,0 +1,12 @@
+package measurex
+
+// NewFailure creates a serializable failure from an error. We
+// cannot round trip an error using JSON, so we serialize to this
+// intermediate format that is a sort of Optional<string>.
+func NewFailure(err error) *string {
+	if err == nil {
+		return nil
+	}
+	s := err.Error()
+	return &s
+}