encompasses all executions in either initialization or running statuses
function getActiveExecution(exId) {
        const str = terminalStatusList().map(state => ` _status:${state} `).join('OR');
        const query = `ex_id: ${exId} NOT (${str.trim()})`;
        return searchExecutionContexts(query, null, 1, '_created:desc')
            .then((ex) => {
                if (ex.length === 0) {
                    const error = new Error(`no active execution context was found for ex_id: ${exId}`);
                    error.code = 404;
                    return Promise.reject(error);
                }
                return ex[0];
            });
    }