[rust-server] Update to error-chain 0.12

[bjgill]

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Description of the PR

Should fix #389. error-chain 0.11 doesn't work with Rust 1.27.0 (new warnings). 0.12 is fully back-compatible - the only change was to silence the new warning

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[bjgill]

The circleCI failure looks unrelated - we don't test rust-server in circleCI.

[jmini]

Well rust was also commented 845df89. Please restore it in this PR so we can see if your change fix the issue or not.

[jmini]

Thank you @bjgill for the quick fix and the revert of 845df89. Travis CI looks good, I have merged the change.