cleanup: doh.powerdns.org is not working anymore (#924)

While there, `.../internal/sessionresolver` => `.../sessionresolver`

See https://github.com/ooni/probe/issues/2255

internal/engine/sessionresolver/errwrapper.go

@@ -0,0 +1,40 @@
+package sessionresolver
+
+//
+// Error wrapping
+//
+
+import (
+	"errors"
+	"fmt"
+)
+
+// errWrapper wraps an error to include the URL of the
+// resolver that we're currently using.
+type errWrapper struct {
+	err error
+	url string
+}
+
+// newErrWrapper creates a new err wrapper.
+func newErrWrapper(err error, URL string) *errWrapper {
+	return &errWrapper{
+		err: err,
+		url: URL,
+	}
+}
+
+// Error implements error.Error.
+func (ew *errWrapper) Error() string {
+	return fmt.Sprintf("<%s> %s", ew.url, ew.err.Error())
+}
+
+// Is allows consumers to query for the type of the underlying error.
+func (ew *errWrapper) Is(target error) bool {
+	return errors.Is(ew.err, target)
+}
+
+// Unwrap returns the underlying error.
+func (ew *errWrapper) Unwrap() error {
+	return ew.err
+}