pkg: Provide a friendlier message when bootstrap fails due to address resolution

The current message when bootstapping pkg fails for any reason implies that pkg
is not available. We have the error code from fetch so if bootstrap failed due
to address resolution say so.

Reviewed by: bapt, bz
Approved by: bz (co-mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D25323