DataStorage: fix typo and actually create $HOME/.config if it doesn't exist yet (#10390)

Fixes #10380.

datastorage.lua

@@ -24,18 +24,18 @@ function DataStorage:getDataDir()
                 lfs.mkdir(os.getenv("XDG_CONFIG_HOME"))
             end
         else
-            local user_rw = jit.os == "OSX" and "Library/Application Support" or ".config"
+            local user_rw = string.format("%s/%s", os.getenv("HOME"), jit.os == "OSX" and "Library/Application Support" or ".config")
             if lfs.attributes(user_rw, "mode") ~= "directory" then
                 lfs.mkdir(user_rw)
             end
-            data_dir = string.format("%s/%s/%s", os.getenv("HOME"), user_rw, "koreader")
+            data_dir = string.format("%s/%s", user_rw, "koreader")
         end
     else
         data_dir = "."
     end
     if lfs.attributes(data_dir, "mode") ~= "directory" then
         local ok, err = lfs.mkdir(data_dir)
-        if not ok then error(err) end
+        if not ok then error(err .. " " .. data_dir) end
     end
 
     return data_dir