Fix bad error message when running as root with invalid $HOME
Fix bad error message when running as root with invalid $HOME

If we're running as root then we check that $HOME is owned by root to
avoid putting root-owned files in a user's home directory. However, if
$HOME doesn't exist then the error was unhelpful:

    # 0install --version
    Unix.Unix_error(Unix.ENOENT, "stat", "/foo")
    (hint: run with OCAMLRUNPARAM=b to get a stack-trace)

Now it says:

    # 0install --version
    warning: Error checking $HOME ("/foo"): Unix.Unix_error(Unix.ENOENT, "stat", "/foo")
    0install (zero-install) 2.15-post

0install/0install,0install/0install,0install/0install