openssl ocsp: make index.txt parsing error more verbose

If index.txt exists but has some problems (like for example consisting of a single \n character in it,
or some field-number error in one of the lines) openssl will just exit without any error message.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from openssl#15360)

apps/ocsp.c

@@ -620,6 +620,9 @@ int ocsp_main(int argc, char **argv)
     if (ridx_filename != NULL) {
         rdb = load_index(ridx_filename, NULL);
         if (rdb == NULL || index_index(rdb) <= 0) {
+            BIO_printf(bio_err,
+                "Problem with index file: %s (could not load/parse file)\n",
+                ridx_filename);
             ret = 1;
             goto end;
         }