emsdk

Avoid post-install when nothing is installed (#736)

This avoids re-running the post-install scripts when commands such as
`./emsdk install latest` a re-run.  This re-running of npm ci can be
significant slowdown especially during testing and developerment.

Becuase of the refactoring this change change also means we exit ealier
when a given tool fails to install.  In general we want to error out as
early as possible on the first failure so as not to bury it.