Ensure that we still return 1, and print something to stderr, on (skopeo copy) failure

https://github.com/projectatomic/skopeo/pull/519 made (skopeo copy)
suceed and print nothing to stderr; that could lead to hard-to-diagnose
failures in rare corner cases, e.g. shell scripts which do
(skopeo copy $src $dst) (as opposed to the correct
(skopeo copy "$src" "$dst") ) if $src and $dst are empty due to
a previous failure.

cmd/skopeo/copy.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -33,7 +34,8 @@ func contextsFromGlobalOptions(c *cli.Context) (*types.SystemContext, *types.Sys
 
 func copyHandler(c *cli.Context) error {
 	if len(c.Args()) != 2 {
-		return cli.ShowCommandHelp(c, "copy")
+		cli.ShowCommandHelp(c, "copy")
+		return errors.New("Exactly two arguments expected")
 	}
 
 	policyContext, err := getPolicyContext(c)