Bump error-chain from 0.12.3 to 0.12.4

Bumps [error-chain](https://github.com/rust-lang-nursery/error-chain) from 0.12.3 to 0.12.4.
- [Release notes](https://github.com/rust-lang-nursery/error-chain/releases)
- [Changelog](https://github.com/rust-lang-nursery/error-chain/blob/master/CHANGELOG.md)
- [Commits](rust-lang-deprecated/error-chain@0.12.3...0.12.4)

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