There is some existing practice in the "terraform" package of returning
a special error type ValidationError from EvalNode implementations in
order to return warnings without halting the graph walk even though a
non-nil error was returned.
This is a diagnostics-flavored version of that approach, allowing us to
avoid totally reworking the EvalNode concept around diagnostics and
retaining the ability to return non-fatal errors.
NonFatalErr is equivalent to the former terraform.ValidationError, while
ErrWithWarnings is a helper that automatically treats any errors as
fatal but returns NonFatalError if the diagnostics contains only warnings.
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