Handle error from ReadOrCreateEncryptedTLSAssets gracefully.

[andrejvanderzee]

Causes panic when ReadOrCreateEncryptedTLSAssets, for example when you enter a bad kmsKeyArn.

Ever considered using Dave Cheney's error package?
https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully

[codecov-io]

Current coverage is 68.99% (diff: 0.00%)

Merging #188 into master will decrease coverage by 0.12%

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[mumoshu]

@andrejvanderzee Thanks as always! LGTM 👍

Regarding the which error package to introduce into kube-aws, I have not yet considered Dave Cheney's one. FYI, reqs for an error package for kube-aws in my mind are supports for (1) nested errors and (2) stack traces for each nested error, and my preference so far is juju/errors. I'm open to suggestion(s) though.