rkt: better error messages when users don't give enough volumes

Example of output:
|  $ rkt --debug --insecure-skip-verify run docker://redis
|  Failed to generate nspawn args: no volume for mountpoint "volume-/data" in app "redis".
|  You can inspect the volumes with:
|  	sudo rkt image cat-manifest --pretty-print sha512-f95f15ce81345214328fdbe9407fdac3e46de0c11ceb69e8da5b0d1aa3158dfc
|  App "redis" requires the following volumes:
|  	--volume volume-/data,kind=host,source=/some/path

Fixes: https://github.com/coreos/rkt/issues/943

stage1/init/init.go

@@ -365,7 +365,7 @@ func stage1() int {
 
 	args, env, err := getArgsEnv(p, flavor, systemdVersion, debug)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Failed to get execution parameters: %v\n", err)
+		fmt.Fprintf(os.Stderr, "%v\n", err)
 		return 3
 	}
 

stage1/init/pod.go

@@ -327,7 +327,16 @@ func (p *Pod) appToNspawnArgs(ra *schema.RuntimeApp, am *schema.ImageManifest) (
 		key := mp.Name
 		vol, ok := vols[key]
 		if !ok {
-			return nil, fmt.Errorf("no volume for mountpoint %q in app %q", key, name)
+			catCmd := fmt.Sprintf("sudo rkt image cat-manifest --pretty-print %v", id)
+			volumeCmd := ""
+			for _, mp := range app.MountPoints {
+				volumeCmd += fmt.Sprintf("--volume %s,kind=host,source=/some/path ", mp.Name)
+			}
+
+			return nil, fmt.Errorf("no volume for mountpoint %q in app %q.\n"+
+				"You can inspect the volumes with:\n\t%v\n"+
+				"App %q requires the following volumes:\n\t%v",
+				key, name, catCmd, name, volumeCmd)
 		}
 		opt := make([]string, 4)
 
@@ -385,7 +394,7 @@ func (p *Pod) PodToNspawnArgs() ([]string, error) {
 		}
 		aa, err := p.appToNspawnArgs(ra, am)
 		if err != nil {
-			return nil, fmt.Errorf("failed to construct args for app %q: %v", am.Name, err)
+			return nil, err
 		}
 		args = append(args, aa...)
 	}