Preserve original error instance

[ro0gr]

This in attempt to improve the san650#640 slightly. Feel free to accept or decline it. I think I'm in general fine to merge your PR as is.

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This adds the cause.error which is supposed to keep the original error instance. So consumers may use it as

  ...
} catch (e) {
  assert.true(e.cause.error instanceof Error)
}

this is probably not the most elegant solution, but looks good enough to me for this rare(maybe I am wrong) usage scenario.

[simonihmig]

Thanks, makes sense!