conformance: fix error msg for DELETE MANIFEST

---
Ran 27 of 27 Specs in 0.120 seconds
SUCCESS! -- 27 Passed | 0 Failed | 0 Pending | 0 Skipped
PASS
---

pkg/api/routes.go

@@ -441,8 +441,11 @@ func (rh *RouteHandler) DeleteManifest(w http.ResponseWriter, r *http.Request) {
 			WriteJSON(w, http.StatusBadRequest,
 				NewErrorList(NewError(NAME_UNKNOWN, map[string]string{"name": name})))
 		case errors.ErrManifestNotFound:
-			WriteJSON(w, http.StatusBadRequest,
+			WriteJSON(w, http.StatusNotFound,
 				NewErrorList(NewError(MANIFEST_UNKNOWN, map[string]string{"reference": reference})))
+		case errors.ErrBadManifest:
+			WriteJSON(w, http.StatusBadRequest,
+				NewErrorList(NewError(UNSUPPORTED, map[string]string{"reference": reference})))
 		default:
 			rh.c.Log.Error().Err(err).Msg("unexpected error")
 			w.WriteHeader(http.StatusInternalServerError)

pkg/compliance/v1_0_0/check.go

@@ -569,7 +569,7 @@ func CheckWorkflows(t *testing.T, config *compliance.Config) {
 			// delete again should fail
 			resp, err = resty.R().Delete(baseURL + "/v2/repo7/manifests/" + digest.String())
 			So(err, ShouldBeNil)
-			So(resp.StatusCode(), ShouldEqual, 400)
+			So(resp.StatusCode(), ShouldEqual, 404)
 
 			// check/get by tag
 			resp, err = resty.R().Head(baseURL + "/v2/repo7/manifests/test:1.0")

pkg/storage/storage.go

@@ -462,10 +462,11 @@ func (is *ImageStore) DeleteImageManifest(repo string, reference string) error {
 		return errors.ErrRepoNotFound
 	}
 
+	// as per spec "reference" can only be a digest and not a tag
 	_, err := godigest.Parse(reference)
 	if err != nil {
 		is.log.Error().Err(err).Msg("invalid reference")
-		return errors.ErrManifestNotFound
+		return errors.ErrBadManifest
 	}
 
 	buf, err := ioutil.ReadFile(path.Join(dir, "index.json"))