More robust team exists api endpoint (#12130)

* More robust team exists api endpoint

* Making the code more concise

* Better handling of errors on GetTeamByName

store/sqlstore/team_store.go

@@ -285,6 +285,9 @@ func (s SqlTeamStore) GetByName(name string) (*model.Team, *model.AppError) {
 
 	err := s.GetReplica().SelectOne(&team, "SELECT * FROM Teams WHERE Name = :Name", map[string]interface{}{"Name": name})
 	if err != nil {
+		if err == sql.ErrNoRows {
+			return nil, model.NewAppError("SqlTeamStore.GetByName", "store.sql_team.get_by_name.missing.app_error", nil, "name="+name+","+err.Error(), http.StatusNotFound)
+		}
 		return nil, model.NewAppError("SqlTeamStore.GetByName", "store.sql_team.get_by_name.app_error", nil, "name="+name+", "+err.Error(), http.StatusInternalServerError)
 	}
 	return &team, nil