Wrap tar errors with some context

Signed-off-by: Silvin Lubecki <silvin.lubecki@docker.com>

Author: silvin-lubecki

internal/packager/packing.go

@@ -2,7 +2,6 @@ package packager
 
 import (
 	"archive/tar"
-	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -12,6 +11,7 @@ import (
 	"github.com/docker/app/internal"
 	"github.com/docker/app/types"
 	"github.com/docker/docker/pkg/archive"
+	"github.com/pkg/errors"
 )
 
 var dockerFile = `FROM docker/cnab-app-base:` + internal.Version + `
@@ -51,26 +51,26 @@ func PackInvocationImageContext(app *types.App, target io.Writer) error {
 		return errors.New("app should have one and only one compose file")
 	}
 	if len(app.ParametersRaw()) != 1 {
-		return errors.New("app should have one and only parameters file")
+		return errors.New("app should have one and only one parameters file")
 	}
 	if err := tarAddBytes(tarout, "Dockerfile", []byte(dockerFile)); err != nil {
-		return err
+		return errors.Wrap(err, "failed to add Dockerfile to the invocation image build context")
 	}
 	if err := tarAddBytes(tarout, ".dockerignore", []byte(dockerIgnore)); err != nil {
-		return err
+		return errors.Wrap(err, "failed to add .dockerignore to the invocation image build context")
 	}
 	if err := tarAddBytes(tarout, prefix+internal.MetadataFileName, app.MetadataRaw()); err != nil {
-		return err
+		return errors.Wrapf(err, "failed to add %q to the invocation image build context", prefix+internal.MetadataFileName)
 	}
 	if err := tarAddBytes(tarout, prefix+internal.ComposeFileName, app.Composes()[0]); err != nil {
-		return err
+		return errors.Wrapf(err, "failed to add %q to the invocation image build context", prefix+internal.ComposeFileName)
 	}
 	if err := tarAddBytes(tarout, prefix+internal.ParametersFileName, app.ParametersRaw()[0]); err != nil {
-		return err
+		return errors.Wrapf(err, "failed to add %q to the invocation image build context", prefix+internal.ParametersFileName)
 	}
 	for _, attachment := range app.Attachments() {
 		if err := tarAdd(tarout, prefix+attachment.Path(), filepath.Join(app.Path, attachment.Path())); err != nil {
-			return err
+			return errors.Wrapf(err, "failed to add attachment %q to the invocation image build context", prefix+attachment.Path())
 		}
 	}
 	return nil