return NGX_HTTP_UPSTREAM_INVALID_HEADER instead of NGX_HTTP_BAD_GATEWAY
to go to a next upstream on invalid_header condition

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 00384a1..acd5c90 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -995,7 +995,7 @@
 
                     for (i = 0; i < flcf->catch_stderr->nelts; i++) {
                         if (ngx_strstr(line.data, pattern[i].data)) {
-                            return NGX_HTTP_BAD_GATEWAY;
+                            return NGX_HTTP_UPSTREAM_INVALID_HEADER;
                         }
                     }
                 }