Add export of FromDecStrErr

[BlinkyStitt]

I ran cargo update on my project and it brought it a new version of uuint (0.6.1). When I tried to run my tests, I got this error:

U256::from_dec_str("10000000000000")?;

the trait `std::convert::From<uint::uint::FromDecStrErr>` is not implemented for `MyError`

I think the uint.rs in this repo should have something like pub use uint::FromDecStrErr so that my code could use the correct version without needing any pins in my Cargo.toml.

For now, I'm pinning uint to 0.4 in my Cargo.toml and my code is building.