r/membership: Fix import crash on incorrect ID

An incorrect two-part ID supplied to github_membership during import
results in a crash - this is because the import functionality is passing
this directly to the Read function of the resource, for which IDs are
programatically controlled and a panic there would be more telling of a
much more serious error worthy of panicking over.

This adds an internal validation function for two-part IDs, and wraps
the resource's import functionality to use it to validate incoming IDs
before passing them to Read.

github/resource_github_membership.go

@@ -2,6 +2,8 @@ package github
 
 import (
 	"context"
+	"errors"
+	"strings"
 
 	"github.com/google/go-github/github"
 	"github.com/hashicorp/terraform/helper/schema"
@@ -15,7 +17,7 @@ func resourceGithubMembership() *schema.Resource {
 		Update: resourceGithubMembershipUpdate,
 		Delete: resourceGithubMembershipDelete,
 		Importer: &schema.ResourceImporter{
-			State: schema.ImportStatePassthrough,
+			State: resourceGithubMembershipImport,
 		},
 
 		Schema: map[string]*schema.Schema{
@@ -89,3 +91,19 @@ func resourceGithubMembershipDelete(d *schema.ResourceData, meta interface{}) er
 
 	return err
 }
+
+func resourceGithubMembershipImport(d *schema.ResourceData, meta interface{}) ([]*schema.ResourceData, error) {
+	// All we do here is validate that the import string is in a correct enough
+	// format to be parsed.  parseTwoPartID will panic if it's missing elements,
+	// and is used otherwise in places where that should never happen, so we want
+	// to keep it that way.
+	id := d.Id()
+	if id == "" {
+		return nil, errors.New("please supply an ID format matching organization:username")
+	}
+	parts := strings.Split(id, ":")
+	if len(parts) < 2 {
+		return nil, errors.New("incorrect ID format. Please supply an ID format matching organization:username")
+	}
+	return []*schema.ResourceData{d}, nil
+}