MM-25118: fix teamImport panic and return better error (#14797)

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

api4/team.go

@@ -1138,6 +1138,7 @@ func importTeam(c *Context, w http.ResponseWriter, r *http.Request) {
 	auditRec.AddMeta("from", importFrom)
 
 	var log *bytes.Buffer
+	data := map[string]string{}
 	switch importFrom {
 	case "slack":
 		var err *model.AppError
@@ -1145,12 +1146,14 @@ func importTeam(c *Context, w http.ResponseWriter, r *http.Request) {
 			c.Err = err
 			c.Err.StatusCode = http.StatusBadRequest
 		}
+		data["results"] = base64.StdEncoding.EncodeToString(log.Bytes())
+	default:
+		c.Err = model.NewAppError("importTeam", "api.team.import_team.unknown_import_from.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	data := map[string]string{}
-	data["results"] = base64.StdEncoding.EncodeToString(log.Bytes())
 	if c.Err != nil {
 		w.WriteHeader(c.Err.StatusCode)
+		return
 	}
 	auditRec.Success()
 	w.Write([]byte(model.MapToJson(data)))