Improve From<ErrorKind> for Error

[data-sync-user]

(From the original https://github.com/mozilla-services/syncstorage-rs/issues/932 )

Mark's reflexive From for Error types in autoendpoint may make it easier to convert from lower level errors (like DbErrorKind) directly into higher level ApiErrors.

We may want to make failure_boilerplate!from_error! implement it for all types or only emit it for the higher level ApiError -- we can look at what autoendpoint does here as an example.

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