[MM-28125] einterface/oauthprovide: return parsing errors (#15426)

* einterface/oauthprovide: return parsing errors

* Update app/oauth.go

* reflect review comments

* fix var name error

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

web/oauth_test.go

@@ -549,10 +549,10 @@ func closeBody(r *http.Response) {
 type MattermostTestProvider struct {
 }
 
-func (m *MattermostTestProvider) GetUserFromJson(data io.Reader) *model.User {
+func (m *MattermostTestProvider) GetUserFromJson(data io.Reader) (*model.User, error) {
 	user := model.UserFromJson(data)
 	user.AuthData = &user.Email
-	return user
+	return user, nil
 }
 
 func GenerateTestAppName() string {