Fix some pending problems (#29985)
These changes are quite independent and trivial, so I don't want to open
too many PRs.
* https://github.com/go-gitea/gitea/pull/29882#discussion_r1529607091
* the `f.Close` should be called properly
* the error message could be more meaningful
(https://github.com/go-gitea/gitea/pull/29882#pullrequestreview-1942557935)
*
https://github.com/go-gitea/gitea/pull/29859#pullrequestreview-1942324716
* the new translation strings don't take arguments
* https://github.com/go-gitea/gitea/pull/28710#discussion_r1443778807
* stale for long time
* #28140
* a form was forgotten to be changed to work with backend code
(cherry picked from commit 226231ea27d4f2b0f09fa4efb39501507613b284)
Conflicts:
templates/repo/issue/view_content/pull.tmpl
discarded because unexplained
templates/status/404.tmpl
implemented differently in Forgejo
d4ea1c2559