Change place of GUNPGHOME for apt-cyg.

In previous version, GNUPGHOME for apt-cyg had been set "$cache/apt-cyg/.gnupg".
But there are some inconveniences.
For example, if you downloaded apt-cyg into "$cache/", GNUPGHOME could not be initialized.
So this patch change GNUPGHOME for apt-cyg from "$cache/apt-cyg/.gnupg" to "$cache/.apt-cyg"

In additionally, a error handler for initializing GNUPGHOME is added.