Migrating Team.SaveMember/SaveMultipleMembers into errors (#14924)

* Migrating Team.SaveMember/SaveMultipleMembers into errors

* Removing copy/paste comments

* Fixing another small thing

* Fixing tests

* Addressing PR review comments

* Fixing some tests

* Fixing golangci-lint

* Fixing linter

* Regenerating store layers

* Fix lint error after merge

* Other lint error fixed

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

app/slashcommands/auto_users.go

@@ -4,8 +4,12 @@
 package slashcommands
 
 import (
+	"errors"
+	"net/http"
+
 	"github.com/mattermost/mattermost-server/v5/app"
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/store"
 	"github.com/mattermost/mattermost-server/v5/utils"
 )
 
@@ -54,8 +58,20 @@ func CreateBasicUser(a *app.App, client *model.Client4) *model.AppError {
 	if err != nil {
 		return err
 	}
-	if _, err = a.Srv().Store.Team().SaveMember(&model.TeamMember{TeamId: basicteam.Id, UserId: ruser.Id}, *a.Config().TeamSettings.MaxUsersPerTeam); err != nil {
-		return err
+	if _, nErr := a.Srv().Store.Team().SaveMember(&model.TeamMember{TeamId: basicteam.Id, UserId: ruser.Id}, *a.Config().TeamSettings.MaxUsersPerTeam); nErr != nil {
+		var appErr *model.AppError
+		var conflictErr *store.ErrConflict
+		var limitExceededErr *store.ErrLimitExceeded
+		switch {
+		case errors.As(nErr, &appErr): // in case we haven't converted to plain error.
+			return appErr
+		case errors.As(nErr, &conflictErr):
+			return model.NewAppError("CreateBasicUser", "app.create_basic_user.save_member.conflict.app_error", nil, nErr.Error(), http.StatusBadRequest)
+		case errors.As(nErr, &limitExceededErr):
+			return model.NewAppError("CreateBasicUser", "app.create_basic_user.save_member.max_accounts.app_error", nil, nErr.Error(), http.StatusBadRequest)
+		default: // last fallback in case it doesn't map to an existing app error.
+			return model.NewAppError("CreateBasicUser", "app.create_basic_user.save_member.app_error", nil, nErr.Error(), http.StatusInternalServerError)
+		}
 	}
 
 	return nil