core: more explanation when a provider block cannot be found

Our new resource-to-provider matching is stricter about explicitly
matching aliases when config is present (no longer automatically
inherited) and with locating providers to destroy removed resources.

With this in mind, this is an attempt to expand slightly on this error
message now that users are more likely to see it.

In future it would be nice to do some explicit validation of this a bit
closer to the UI, so we can have room for more explanatory text, but this
additional messaging is intended to help users understand why they might
be seeing this message after removing a provider configuration block from
configuration, whether directly or as a side-effect of removing a module.

terraform/transform_provider.go

@@ -101,8 +101,9 @@ func (t *ProviderTransformer) Transform(g *Graph) error {
 
 			if target == nil {
 				err = multierror.Append(err, fmt.Errorf(
-					"%s: provider %s couldn't be found",
-					dag.VertexName(v), p))
+					"%s: configuration for %s is not present; a provider configuration block is required for all operations",
+					dag.VertexName(v), p,
+				))
 				break
 			}