pass: return an error when a cred doesn't exist

fixes https://github.com/docker/docker-credential-helpers/issues/174

Signed-off-by: Nick Santos <nick.santos@docker.com>

pass/pass.go

@@ -138,7 +138,7 @@ func (p Pass) Get(serverURL string) (string, string, error) {
 
 	if _, err := os.Stat(path.Join(getPassDir(), PASS_FOLDER, encoded)); err != nil {
 		if os.IsNotExist(err) {
-			return "", "", nil
+			return "", "", credentials.NewErrCredentialsNotFound()
 		}
 
 		return "", "", err

pass/pass_test.go

@@ -54,13 +54,9 @@ func TestPassHelper(t *testing.T) {
 			t.Fatal(err)
 		}
 
-		username, _, err = helper.Get(server)
+		_, _, err = helper.Get(server)
-		if err != nil {
+		if !credentials.IsErrCredentialsNotFound(err) {
-			t.Fatal(err)
+			t.Fatalf("expected credentials not found, actual: %v", err)
-		}
-
-		if username != "" {
-			t.Fatalf("%s shouldn't exist any more", username)
 		}
 	}
 
@@ -73,3 +69,12 @@ func TestPassHelper(t *testing.T) {
 		t.Fatal("didn't delete all creds?")
 	}
 }
+
+func TestMissingCred(t *testing.T) {
+	helper := Pass{}
+
+	_, _, err := helper.Get("garbage")
+	if !credentials.IsErrCredentialsNotFound(err) {
+		t.Fatalf("expected credentials not found, actual: %v", err)
+	}
+}