Fix: require.async module exception delegation

The fs.readFile bug was hiding another bug that was causing this test
to pass, even so it was broken:

require.async("../fixtures/throws_error1") in test-module-loading.js

This patch fixes the original test by running _compile within a
try..catch block for _loadScript.

_loadScriptSync also had some useless (deprecated?) code for dealing
with module entry point exceptions. This code was also removed for
greater clarity.