code: 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];
              });
      }
docs: encompasses all executions in either initialization or running statuses