Suppress yarnpkg is not a command when only npm is installed

[anp]

Inheriting stdout is how CRNA shows init status to the developer, but maybe it should be buffered until we're sure that yarn is installed for the "attempt installing with yarn first" version. Or maybe it's OK to just show this -- there's a lot of other output users see immediately after.

[gaearon]

FWIW this is what we do in CRA.

[anp]

@gaearon thanks! Very useful.