Fix installer logging/error reporting.

Due to the GUI using stderr to show upgrade failures we have problems
if we output "set -x" output from the install scripts and other
verbosity to stderr.

Instead use a bounce script to put the stderr of the old install_worker
into a temp file and have the install script use file descriptor

This allows us to debug the installer when it goes upside down, but
also preserve the old behavior and have it be less chatty.