Fix Rust 1.42 (beta) failures in Travis CI

[mulkieran]

Rust 1.41 has been released and we're experiencing some beta failures.

[mulkieran]

It looks like error_chain will cut a new release which will cause the error_chain lints to go away:
rust-lang-deprecated/error-chain#285. So we should leave those alone, and just address the non error_chain problems.

[mulkieran]

Whoops! These errors have to do with Rust error management and the Error trait, so we made need to figure out a broader fix. We define description in our own errors in stratisd and devicemapper.

[mulkieran]

Resolved by attached PRs.