Handle and propagate errors when checking if paths are Dirs, Files or Exist (#13186)

Handle and propagate errors when checking if paths are Dirs, Files or Exist (#13186)

* Ensure errors from IsDir propagate

* Handle errors when checking IsFile

* Handle and propagate errors from IsExist

* Update modules/templates/static.go

* Update modules/templates/static.go

* Return after ctx.ServerError

* Apply suggestions from code review

* Fix tests

The previous merge managed to break repo_form.go

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lauris BH <lauris@nix.lv>