Issue 44 get rid of error chain dependency

[tommady]

fix #44

Hi, I am a user of this wonderful library,
and currently, I try to use anyhow into my application but causing this error

E0277: `(dyn std::error::Error + std::marker::Send + 'static)` cannot be shared between threads safely  `(dyn std::e
rror::Error + std::marker::Send + 'static)` cannot be shared between threads safely  help: the trait `std::marker::S
ync` is not implemented for `(dyn std::error::Error + std::marker::Send + 'static)` note: required because of the re
quirements on the impl of `std::marker::Sync` for `std::ptr::Unique<(dyn std::error::Error + std::marker::Send + 'st
atic)>` note: required because it appears within the type `std::boxed::Box<(dyn std::error::Error + std::marker::Sen
d + 'static)>` note: required because it appears within the type `std::option::Option<std::boxed::Box<(dyn std::erro
r::Error + std::marker::Send + 'static)>>` note: required because it appears within the type `error_chain::State` no
te: required because it appears within the type `ta::errors::Error` note: required because of the requirements on th
e impl of `std::convert::From<ta::errors::Error>` for `anyhow::Error` note: required by `std::convert::From::from`

and seems like that error is based on this issue from error-chain

then I noticed that you opened an issue try to get rid of that library,
so that's why this PR happened.

hope you will accept it.
thank you.

[greyblake]

@tommady , thank you!