Format should not panic if you try to format it more than once

[zxqfl]

Some logging frameworks will format things sent to the error! macro twice.

Some possible solutions:

1. Print a "iterator exhausted" message instead of panicking
2. Impose a Clone bound on the iterator being formatted so that we can format it multiple times
3. Doesn't fmt return a Result anyway? Why not just return Err?

[bluss]

(3) From previous discussion I have understood that fmt's error return should not be used by the thing that is being formatted. It's used to relay the information that an I/O error happened through the formatting chain.

I don't know how to solve it, I think this is a drawback that comes from our skirting the rules a bit for formatting.