@Override
            public boolean select(Metadata metadata) {
                if (maxEmbedded < 0) {
                    return true;
                }
                return embedded++ < maxEmbedded;
            }
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