This provides an explicit API for call-fail-retry-succeed evaluation
flows, such as currently used in NixOps4.
An alternative design would simply reset the `Value` to the original
thunk instead of `tFailed` under the condition of catching a
`RecoverableEvalError`.
That is somewhat simpler, but I believe the presence of `tFailed` is
beneficial for possible use in the repl; being able to show the error
sooner, without re-evaluation.
The hasPos method is required in order to avoid an include loop.
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