private String getContainerStacktrace(FetchEmitTuple t, List<Metadata> metadataList) {
        if (metadataIsEmpty(metadataList)) {
            return StringUtils.EMPTY;
        }
        String stack = metadataList.get(0).get(TikaCoreProperties.CONTAINER_EXCEPTION);
        return (stack != null) ? stack : StringUtils.EMPTY;
    }
returns stack trace if there was a container exception or empty string
if there was no stacktrace

@param t
@param metadataList
@return
tika-core/src/main/java/org/apache/tika/pipes/PipesServer.java