Add error logs if findMainPackageName errors out
Add error logs if findMainPackageName errors out

[#167105792]

Co-authored-by: David Freilich <0106e5ee349bb58d7c51800b6acf5b57cf80aef6@pivotal.io>

cloudfoundry/go-buildpack,cloudfoundry/go-buildpack,cloudfoundry/go-buildpack