set of.failed for cached error, the bug has been introduced in r2757

src/core/ngx_open_file_cache.c

diff --git a/src/core/ngx_open_file_cache.c b/src/core/ngx_open_file_cache.c
index e0b434b..173980f 100644
--- a/src/core/ngx_open_file_cache.c
+++ b/src/core/ngx_open_file_cache.c
@@ -234,6 +234,7 @@
 
             } else {
                 of->err = file->err;
+                of->failed = ngx_open_file_n;
             }
 
             goto found;