[Distutils] A possible refactor/streamlining of PEP 517

[Donald Stufft]

Fwiw I also agree return values are more robust, though the fact that an exception has a message is also a nice property since it allows backends to provide a reason they dont support sdist in this tree (which some tools may want to surface to users, particularly if they *need* a sdist, like a hypothetical 'twine sdist' command). That could be implemented by returning a Tuple though or something. 

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> On Jul 17, 2017, at 9:12 AM, Thomas Kluyver <thomas at kluyver.me.uk> wrote:
> 
>> 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.