Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

secretservice/secretservice_linux.go

@@ -74,7 +74,7 @@ func (h Secretservice) Get(serverURL string) (string, string, error) {
 	user := C.GoString(username)
 	pass := C.GoString(secret)
 	if pass == "" {
-		return "", "", credentials.ErrCredentialsNotFound
+		return "", "", credentials.NewErrCredentialsNotFound()
 	}
 	return user, pass, nil
 }

secretservice/secretservice_linux_test.go

@@ -43,7 +43,7 @@ func TestMissingCredentials(t *testing.T) {
 
 	helper := Secretservice{}
 	_, _, err := helper.Get("https://adsfasdf.wrewerwer.com/asdfsdddd")
-	if err != credentials.ErrCredentialsNotFound {
-		t.Fatalf("exptected ErrCredentialsNotFound, got %v", err)
+	if !credentials.IsErrCredentialsNotFound(err) {
+		t.Fatalf("expected ErrCredentialsNotFound, got %v", err)
 	}
 }