Print errors from __gc finalizers instead of rethrowing them.

Finalizers are not supposed to throw errors -- this is undefined behavior.
Lua 5.1 - 5.3 and (previously) LuaJIT rethrow the error. This randomly
breaks some unrelated code that just happens to do an allocation. Bad.
Lua 5.4 catches the error and emits a warning instead. But warnings are
not enabled by default, so it fails silently. Even worse.
LuaJIT (now) catches the error and emits a VM event. The default event
handler function prints "ERROR in finalizer: ...".
Set a custom handler function with: jit.attach(handler, "errfin")

src/lj_gc.c

@@ -27,6 +27,7 @@
 #include "lj_trace.h"
 #include "lj_dispatch.h"
 #include "lj_vm.h"
+#include "lj_vmevent.h"
 
 #define GCSTEPSIZE	1024u
 #define GCSWEEPMAX	40
@@ -521,8 +522,13 @@ static void gc_call_finalizer(global_State *g, lua_State *L,
   hook_restore(g, oldh);
   if (LJ_HASPROFILE && (oldh & HOOK_PROFILE)) lj_dispatch_update(g);
   g->gc.threshold = oldt;  /* Restore GC threshold. */
-  if (errcode)
-    lj_err_throw(L, errcode);  /* Propagate errors. */
+  if (errcode) {
+    ptrdiff_t errobj = savestack(L, L->top-1);  /* Stack may be resized. */
+    lj_vmevent_send(L, ERRFIN,
+      copyTV(L, L->top++, restorestack(L, errobj));
+    );
+    L->top--;
+  }
 }
 
 /* Finalize one userdata or cdata object from the mmudata list. */