Let Stat() have an err outparam instead of writing to stderr.

Let Stat() have an err outparam instead of writing to stderr.

Also check for Stat() failure in a few more places.

This way, ninja doesn't print two "ninja: error: " lines if stat() fails
during a build.  It also makes it easier to keep the stat tests quiet.
Every caller of Stat() needs to explicitly log the error string if
that's desired.