[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>

einterfaces/oauthproviders.go

@@ -10,7 +10,7 @@ import (
 )
 
 type OauthProvider interface {
-	GetUserFromJson(data io.Reader) *model.User
+	GetUserFromJson(data io.Reader) (*model.User, error)
 }
 
 var oauthProviders = make(map[string]OauthProvider)