make.go: don't treat all ReadFile errors the same

Change-Id: Icdb2f313e77f46fba6e094cab9ae6a114c3bcef2
make.go, genfileembed: track genfileembed output files and delete stale ones

Don't just trust all zembed_*.go files to be wanted. Instead, we now
ask genfileembed to write out its output files to stderr, parse that,
and add those to our set of wanted files. Then when we look for
unwanted files that aren't known outputs that now also potentially
includes zembed_*.go files.

This fixes a JavaScript breakage from when Aaron renamed some
JavaScript files recently.

Turns out I had 4 such files that were stale outputs.

In other news, I hereby declare a moratorium on updates to make.go.
It has become disgusting and is need of cleanup love before it is
disgraced further.

Change-Id: Ia1432d21ce486bfbca9e3c9db6f1852b859ebfc0