Fix undue asserts in dmon_extra

Some asserts are placed in case that can effectively occur
so we remove the assertion and we return false. In turn we
adapt the logic accordingly so when false is returned to add
a watch we do not open that directory.

data/core/init.lua

@@ -224,14 +224,14 @@ end
 
 
 function core.project_subdir_set_show(dir, filename, show)
-  dir.shown_subdir[filename] = show
   if dir.files_limit and not dir.force_rescan then
     local fullpath = dir.name .. PATHSEP .. filename
-    local success = (show and system.watch_dir_add or system.watch_dir_rm)(dir.watch_id, fullpath)
-    if not success then
-      core.log("Internal warning: error calling system.watch_dir_%s", show and "add" or "rm")
+    if not (show and system.watch_dir_add or system.watch_dir_rm)(dir.watch_id, fullpath) then
+      return false
     end
   end
+  dir.shown_subdir[filename] = show
+  return true
 end