refactor: make measurex depend on measurexlite (#892)

This diff makes `measurex` depend on `measurexlite` rather than the other way around.

While there, add unit tests.

Closes https://github.com/ooni/probe/issues/2240

internal/measurexlite/failure.go

@@ -0,0 +1,37 @@
+package measurexlite
+
+import (
+	"errors"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+)
+
+// NewFailure creates an OONI failure from an error. If the error is nil,
+// we return nil. If the error is not already an ErrWrapper, it's converted
+// to an ErrWrapper. If the ErrWrapper's Failure is not empty, we return its
+// string representation. Otherwise we return a string indicating that
+// an ErrWrapper has an empty failure (should not happen).
+//
+// See https://github.com/ooni/spec/blob/master/data-formats/df-007-errors.md
+// for more information about OONI failures.
+func NewFailure(err error) *string {
+	if err == nil {
+		return nil
+	}
+	// The following code guarantees that the error is always wrapped even
+	// when we could not actually hit our code that does the wrapping. A case
+	// in which this could happen is with context deadline for HTTP when you
+	// have wrapped the underlying dialers but not the Transport.
+	var errWrapper *netxlite.ErrWrapper
+	if !errors.As(err, &errWrapper) {
+		err := netxlite.NewTopLevelGenericErrWrapper(err)
+		couldConvert := errors.As(err, &errWrapper)
+		runtimex.PanicIfFalse(couldConvert, "we should have an ErrWrapper here")
+	}
+	s := errWrapper.Failure
+	if s == "" {
+		s = "unknown_failure: errWrapper.Failure is empty"
+	}
+	return &s
+}