Introduce SyncError
Introduce SyncError

SyncError provides adaption to wrap around error_chain, both where we
use it, and if any libraries rustup is using use it. Once introduced we
can start incrementally migrating function and error at at time to
anyhow without requiring a single big bang commit.

nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,rust-lang/rustup,rust-lang/rustup