Num::FromStrRadixErr should have Trait bound

[cuviper]

From @jethrogb on April 14, 2016 4:41

Num::FromStrRadixErr should be bound by std::error::Error or at least std::fmt::Debug, for use in various Result methods.

Copied from original issue: rust-num/num#185

[cuviper]

I've decided to limit the clutter of breaking change issues, so they will now be tracked collectively in #47. I'm closing this issue for now as postponed but we can still discuss its merits here as needed.