Make errors usable across threads

[rkarp]

Will also allow the blanket impl for the Fail trait of the failure crate to work.

[kubo]

@rkarp Thank you! I'll release rust-oracle 0.2.0 after some other breaking changes are added.

[rkarp]

@kubo BTW, I don't believe this is a breaking change since it just implicitly lets oracle::Error implement Send + Sync.

Edit: Well, except if someone explicitly requires his errors to be !Send or !Sync which seems extremely unlikely.

[kubo]

It introduced additional restriction to the argument of oracle::Error::ParseError.
If someone writes oracle::Error::ParseError(Box::new(err)) and the err doesn't implement Send + Sync, it cannot be compiled. So I consider it is a breaking change even though I believe that nobody writes such code.