command/init: Better error message for provider unsupported platform
command/init: Better error message for provider unsupported platform

As we add and remove support for different target platforms over time,
there will be transition periods where the available platforms for
each provider will be different than the available platforms for Terraform
CLI itself.

In recognition of that possibility, here we add a more specialized error
message for that situation which tries to explain the problem a little
more clearly than the generic error message that came before it.

In an ideal world we'd be able to detect situations where a newer or older
version has support in a similar vein to what we do with provider protocol
incompatibilities, but we don't currently have logic to fetch the data
necessary to implement that, so this is instead a presentation-only change
aimed at allowing some possible near-future changes to the supported
target platforms.

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