MM-30041: Return correct error message for user save (#16117)

* MM-30041: Return correct error message for user save

We were collapsing all types of user conflict into a single
error message. Fixed it by inspecting the field of the invalidError type
and returning the correct message.

https://mattermost.atlassian.net/browse/MM-30041

```release-note
NONE
```

* Fix test errors

* Fix wrong message in test when comparing error messages

Co-authored-by: Rodrigo Villablanca <villa061004@gmail.com>

app/user.go

@@ -306,7 +306,14 @@ func (a *App) createUser(user *model.User) (*model.User, *model.AppError) {
 		case errors.As(nErr, &appErr):
 			return nil, appErr
 		case errors.As(nErr, &invErr):
-			return nil, model.NewAppError("createUser", "app.user.save.existing.app_error", nil, invErr.Error(), http.StatusBadRequest)
+			switch invErr.Field {
+			case "email":
+				return nil, model.NewAppError("createUser", "app.user.save.email_exists.app_error", nil, invErr.Error(), http.StatusBadRequest)
+			case "username":
+				return nil, model.NewAppError("createUser", "app.user.save.username_exists.app_error", nil, invErr.Error(), http.StatusBadRequest)
+			default:
+				return nil, model.NewAppError("createUser", "app.user.save.existing.app_error", nil, invErr.Error(), http.StatusBadRequest)
+			}
 		default:
 			return nil, model.NewAppError("createUser", "app.user.save.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 		}