refactor(sessionresolver): minor changes in files and types naming (#810)

Part of https://github.com/ooni/probe/issues/2135

internal/engine/internal/sessionresolver/errwrapper.go

@@ -1,23 +1,40 @@
 package sessionresolver
 
+//
+// Error wrapping
+//
+
 import (
 	"errors"
 	"fmt"
 )
 
-// errwrapper wraps an error to include the URL of the
+// errWrapper wraps an error to include the URL of the
 // resolver that we're currently using.
-type errwrapper struct {
-	error
-	URL string
+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.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.error, target)
+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
 }