move yarn check to front of installation and improve failure message

Currently the system will install the base component system plus hotloader then try installing webpacker. At this point it checks for yarn, and if yarn is not there bombs out.

It would be better to check at the beginning. And if yarn is not available suggest installing without webpacker.

Trace the installation flow that installs the base component system, hotloader, and webpacker, and locate the existing yarn check. Move that check to the beginning and update the failure message to suggest installing without webpacker; verify the installer fails with the intended guidance when yarn is unavailable.

Written by the indexing model from the issue text.