[Distutils] A possible refactor/streamlining of PEP 517

[Thomas Kluyver]

On Mon, Jul 17, 2017, at 02:56 PM, Nick Coghlan wrote:
> If Thomas is OK with it, I'd be fine with using "raise
> NotImplementedError" for that purpose.

I will implement this if it's what we decide, but I agree with Nathaniel
that a sentinel value is probably more robust, because return values
can't automatically bubble up from internal code like errors do.