Use the correct error variable from provider.GetSSOSettings (#18920)

```release-note
NONE
```

app/oauth.go

@@ -774,7 +774,7 @@ func (a *App) AuthorizeOAuthUser(w http.ResponseWriter, r *http.Request, service
 
 	sso, e2 := provider.GetSSOSettings(a.Config(), service)
 	if e2 != nil {
-		return nil, "", nil, nil, model.NewAppError("AuthorizeOAuthUser.GetSSOSettings", "api.user.get_authorization_code.endpoint.app_error", nil, e.Error(), http.StatusNotImplemented)
+		return nil, "", nil, nil, model.NewAppError("AuthorizeOAuthUser.GetSSOSettings", "api.user.get_authorization_code.endpoint.app_error", nil, e2.Error(), http.StatusNotImplemented)
 	}
 
 	b, strErr := b64.StdEncoding.DecodeString(state)

app/oauth_test.go

@@ -6,6 +6,7 @@ package app
 import (
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
@@ -14,7 +15,10 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/einterfaces/mocks"
 	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin/plugintest/mock"
 )
 
 func TestGetOAuthAccessTokenForImplicitFlow(t *testing.T) {
@@ -447,6 +451,24 @@ func TestAuthorizeOAuthUser(t *testing.T) {
 		assert.Equal(t, "oauth.gitlab.tos.error", err.Id)
 	})
 
+	t.Run("with error in GetSSOSettings", func(t *testing.T) {
+		th := setup(t, true, true, true, "")
+		defer th.TearDown()
+
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.OpenIdSettings.Enable = true
+		})
+
+		providerMock := &mocks.OAuthProvider{}
+		providerMock.On("GetSSOSettings", mock.Anything, model.ServiceOpenid).Return(nil, errors.New("error"))
+		einterfaces.RegisterOAuthProvider(model.ServiceOpenid, providerMock)
+
+		_, _, _, _, err := th.App.AuthorizeOAuthUser(nil, nil, model.ServiceOpenid, "", "", "")
+		require.NotNil(t, err)
+		assert.Equal(t, "api.user.get_authorization_code.endpoint.app_error", err.Id)
+
+	})
+
 	t.Run("enabled and properly configured", func(t *testing.T) {
 		testCases := []struct {
 			Description                   string