We usually use Nifi Content and Provenance repository to troubleshoot failed flow files. Both are set to 7 days of retention. Plus you can replay content to debug.
From zero API reporting perspective you can use row counts as attributes collecting total rows, success rows, failed rows etc. Depending on format of your source file this can be as simple as executing wc -l. Later converting these attributes to JSON and use MergeContent, Schema registry and query record to create an email report.