magit-list-repos: skip unaccessible directories when searching for repositories

When magit-repo-dirs refers to directories user has no access to, it's
impossible to use prefixed form of M-x magit-status due to 'permission
denied' error:

    Debugger entered--Lisp error: (file-error "Opening directory" "permission denied" "/home/user/.unaccessible")
      directory-files("/home/user/.unaccessible" t nil t)
      magit-list-repos*("/home/user/.unaccessible" 2)
      magit-list-repos*("~/" 3)
      magit-list-repos(("~/src/git-mirrors/" "~/"))
      ...

Signed-off-by: Mykola Nikishov <mn@mn.com.ua>