Estimated end sometimes has strange Debug ouput in it

[joshtriplett]

Estimated end: TimeDelta { secs: -1, nanos: 884994194 }

[Skgland]

I think this is related to #660.

Due to the completed crates being more than the total crates the calculated remaining crates are negative, which results in a negative remaining time being calculated.
A negative chrono::TimeDelta can't be represented by the std Duration type so the conversion fails and it falls back to debug printing in https://github.com/rust-lang/crater/blob/master/src/server/routes/ui/experiments.rs#L134