core: Fix TestContextImport_collision

The previous commit rewrote this incorrectly because the fatal message
made it seem like it was failing when an error occurs, but an error is
actually expected here.

Also includes a more detailed error message for this case, consistent with
our new diagnostics style.

terraform/context_import_test.go

@@ -137,8 +137,8 @@ func TestContextImport_collision(t *testing.T) {
 			},
 		},
 	})
-	if diags.HasErrors() {
-		t.Fatalf("unexpected errors: %s", diags.Err())
+	if !diags.HasErrors() {
+		t.Fatalf("succeeded; want an error indicating that the resource already exists in state")
 	}
 
 	actual := strings.TrimSpace(state.String())

terraform/transform_import_state.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/hashicorp/terraform/addrs"
+	"github.com/hashicorp/terraform/tfdiags"
 )
 
 // ImportStateTransformer is a GraphTransformer that adds nodes to the
@@ -96,6 +97,8 @@ func (n *graphNodeImportState) EvalTree() EvalNode {
 // resources they don't depend on anything else and refreshes are isolated
 // so this is nearly a perfect use case for dynamic expand.
 func (n *graphNodeImportState) DynamicExpand(ctx EvalContext) (*Graph, error) {
+	var diags tfdiags.Diagnostics
+
 	g := &Graph{Path: ctx.Path()}
 
 	// nameCounter is used to de-dup names in the state.
@@ -132,20 +135,27 @@ func (n *graphNodeImportState) DynamicExpand(ctx EvalContext) (*Graph, error) {
 	for _, addr := range addrs {
 		result, err := filter.Filter(addr.String())
 		if err != nil {
-			return nil, fmt.Errorf("Error verifying address %s: %s", addr, err)
+			diags = diags.Append(fmt.Errorf("Error while checking for existing %s in state: %s", addr, err))
+			continue
 		}
 
 		// Go through the filter results and it is an error if we find
 		// a matching InstanceState, meaning that we would have a collision.
 		for _, r := range result {
-			if _, ok := r.Value.(*InstanceState); ok {
-				return nil, fmt.Errorf(
-					"Can't import %s, would collide with an existing resource.\n\n"+
-						"Please remove or rename this resource before continuing.",
-					addr)
+			if is, ok := r.Value.(*InstanceState); ok {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Resource already managed by Terraform",
+					fmt.Sprintf("Terraform is already managing a remote object for %s, with the id %q. To import to this address you must first remove the existing object from the state.", addr, is.ID),
+				))
+				continue
 			}
 		}
 	}
+	if diags.HasErrors() {
+		// Bail out early, then.
+		return nil, diags.Err()
+	}
 
 	// For each of the states, we add a node to handle the refresh/add to state.
 	// "n.states" is populated by our own EvalTree with the result of
@@ -166,7 +176,7 @@ func (n *graphNodeImportState) DynamicExpand(ctx EvalContext) (*Graph, error) {
 	}
 
 	// Done!
-	return g, nil
+	return g, diags.Err()
 }
 
 // graphNodeImportStateSub is the sub-node of graphNodeImportState