command/init: Specialized error message for host that isn't registry
command/init: Specialized error message for host that isn't registry

If the user specifies a host that isn't a provider registry in a provider
source address then we'll print out some specialized error messages for
different variants of that situation.

In particular, this includes a special case for when the error is on the
hostname "github.com", in anticipation of folks incorrectly attempting to
use GitHub repository URLs (or Go-style module paths that happen to be
on GitHub) to specify providers, so we can give a more specific hint about
that.

This is just a different presentation of an existing error case that we
are already covering in the installer tests, so there are no new tests
here. We could in principle have a test covering the exact text of these
error messages, but we don't have much precedent for command package tests
covering that level of cosmetic detail.

ewbankkit/terraform,ewbankkit/terraform,ewbankkit/terraform,ewbankkit/terraform,tommynsong/terraform,hashicorp/terraform,pdecat/terraform,pdecat/terraform,hashicorp/terraform,rnaveiras/terraform,tommynsong/terraform,rnaveiras/terraform,apparentlymart/terraform,apparentlymart/terraform,hashicorp/terraform,apparentlymart/terraform,pdecat/terraform,rnaveiras/terraform,pdecat/terraform,apparentlymart/terraform,tommynsong/terraform,apparentlymart/terraform