refine slow warning

[JohannesBuchner]

At the moment the warning that ultranest is progressing slowly is giving a few pointers.

However, the frac_remain one is not so useful, if the percentage is still zero. Perhaps suggesting max_iters (and maybe the number of iteration already) reached may be better?

Maybe we can point to a web tutorial as well?

Also it is writing to a file whose name contains a unresolved %d