Restore the old API key code

[sethvargo]

Semi-revert of GH-239. This adds two "error" fields to our responses, but it's the best interim solution I can think of.

/assign @mikehelmick

Release Note

Temporarily return `error_code` and `errorCode` in API JSON responses for backwards compatibility. `error_code` will be removed in a future version.

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[sethvargo]

/hold

See chat

[mikehelmick]

this is fine - it will be essentially impossible to deprecate.
If we ever create a v2 of the API, we can really remove it.

/lgtm
/unhold