Revert "Ignore "ln" errors"

This reverts commit 7b8f614b09.

autoload/startify.vim

@@ -864,6 +864,9 @@ function! s:create_last_session_link(spath)
           \ shellescape(fnamemodify(a:spath, ':t')),
           \ shellescape(s:session_dir .'/__LAST__'))
     silent! call system(cmd)
+    if v:shell_error
+      echomsg "startify: Can't create 'last used session' symlink."
+    endif
   endif
 endfunction