appc: fix actool build error

app-container/aci/layout.go

@@ -2,9 +2,9 @@ package aci
 
 /*
 
-Filesystem Layout
+Image Layout
 
-The on-disk format is straightforward, with a rootfs and an app manifest.
+The on-disk layout of an app container is straightforward. It includes a rootfs with all of the files that will exist in the root of the app and an app manifest describing how to execute the app.
 
 /app
 /rootfs

app-container/actool/build.go

@@ -81,7 +81,8 @@ func runBuild(args []string) (exit int) {
 	tgt := args[1]
 	ext := filepath.Ext(tgt)
 	if ext != schema.ACIExtension {
-		stderr("fileset: Extension must be %s was %s", schema.ACIExtension, ext)
+		stderr("fileset: Extension must be %s (given %s)", schema.ACIExtension, ext)
+		return 1
 	}
 
 	fsm := schema.NewFileSetManifest(buildName)

app-container/fileset/fileset.go

@@ -9,6 +9,7 @@ import (
 	"github.com/coreos-inc/rkt/app-container/schema"
 )
 
+// ArchiveWriter builds a fileset app container image
 type ArchiveWriter struct {
 	*tar.Writer
 	manifest *schema.FileSetManifest