Add the throw_safe variable to the uv_fs_s struct

This variable is meant to control whether IO errors are created or not.
This enables: nodejs/node-v0.x-archive#8189

The general idea is to throw less errors that are immediately caught.

include/uv.h

@@ -1906,6 +1906,7 @@ struct uv_fs_s {
   uv_fs_type fs_type;
   uv_loop_t* loop;
   uv_fs_cb cb;
+  int throw_safe;
   ssize_t result;
   void* ptr;
   const char* path;