Fixes error interpolation case where err is nil

postal/service.go

@@ -1,6 +1,7 @@
 package postal
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"path/filepath"
@@ -179,7 +180,7 @@ func (s Service) Deliver(dependency Dependency, cnbPath, layerPath, platformPath
 	}
 
 	if !ok {
-		return fmt.Errorf("checksum does not match: %s", err)
+		return errors.New("failed to validate dependency: checksum does not match")
 	}
 
 	return nil

postal/service_test.go

@@ -616,7 +616,7 @@ version = "this is super not semver"
 						"",
 					)
 
-					Expect(err).To(MatchError(ContainSubstring("checksum does not match")))
+					Expect(err).To(MatchError("validation error: checksum does not match"))
 				})
 			})