Return user friendly error when package doesn't exist

pkg/apiserver/registry/datapackaging/package_storage_client.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"encoding/base32"
 	"fmt"
+	"regexp"
 	"strings"
 
 	internalpkgingv1alpha1 "github.com/vmware-tanzu/carvel-kapp-controller/pkg/apis/internalpackaging/v1alpha1"
@@ -152,7 +153,25 @@ func (t PackageTranslator) ToExternalWatcher(intObjWatcher watch.Interface, fiel
 }
 
 func (t PackageTranslator) ToExternalError(err error) error {
-	// TODO: implement
+	if errors.IsNotFound(err) {
+		errStr := err.Error()
+		regEx := `^internalpackages\.internal\.packaging\.carvel\.dev "(?P<internalPackageName>[a-z0-9]*)" not found$`
+		re := regexp.MustCompile(regEx)
+		match := re.FindStringSubmatch(errStr)
+		internalPackageName := ""
+		for i, name := range re.SubexpNames() {
+			if i != 0 && name == "internalPackageName" {
+				internalPackageName = match[i]
+			}
+		}
+
+		packageName, err := t.ToExternalName(internalPackageName)
+		if err != nil {
+			return err
+		}
+		return errors.NewNotFound(datapkgingv1alpha1.Resource("package"), packageName)
+	}
+
 	return err
 }