Replace etaoin.impl.util/error with Slingshot throw+

[dgr]

Closes #666

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• I have read CONTRIBUTING and the Etaoin Developer Guide.

• This PR corresponds to an issue that the Etaoin maintainers have agreed to address. Not yet.

• This PR contains test(s) to protect against future regressions. Not applicable.

• I have updated CHANGELOG.adoc with a description of the addressed issue.

[lread]

I'm not sure this change always conveying more information about the exception.
I mean :type :etaoin/failed is pretty generic.
But when Etaoin throws, you are being more consistent in the shape of the thing it throws.

I commented in the issue that this could be technically breaking, but probably not worth worrying about. Folks should not be relying on exception messages, and even if they are, you've not changed them.

I appreciate all the attention you are giving to Etaoin but this is on the nitty-gritty level that I don't really care about for a project like Etaoin. It feels like cleanup for cleanup's sake, and I'm not sure who it really benefits.

It does give a clear message to developers of Etaoin they should be using throw+ when throwing, so there's that.

And I guess it won't hurt any users of Etaoin.