specfile: raise when missing rpm lib in expand_macro()
specfile: raise when missing rpm lib in expand_macro()

The expand_macro() method uses the rpm library. Prior to this change, if
the rpm lib is not available, rdopkg will crash with a hard-to-decipher
error.

We were able to trigger this crash when running `rdopkg pkgenv` with
rdopkg in a virtualenv.

Print a nicer error to the user when this happens.

Change-Id: I765d688e708e22f3b11aa13b5ba285eecf96d484

openstack-packages/rdopkg,redhat-openstack/rdopkg,openstack-packages/rdopkg,redhat-openstack/rdopkg