Made project_scan_thread() ignore directories it can't open

Should fix #30 and #7

data/core/init.lua

@@ -27,7 +27,7 @@ local function project_scan_thread()
     coroutine.yield()
     t = t or {}
     local size_limit = config.file_size_limit * 10e5
-    local all = system.list_dir(path)
+    local all = system.list_dir(path) or {}
     local dirs, files = {}, {}
 
     for _, file in ipairs(all) do