catch command-not-found errors

Add a special handler to catch errors caused by specifying unknown
commands to Podman.  This allows printing a more helpful error message.

```
$ podman
Command "123123" not found.
See `podman --help`.

$ podman pod 123123
Command "123123" not found.
See `podman pod --help`.
```

Signed-off-by: Valentin Rothberg <vrothberg@suse.com>

Closes: #1379
Approved by: rhatdan

cmd/podman/main.go

@@ -52,6 +52,7 @@ func main() {
 	app.Name = "podman"
 	app.Usage = "manage pods and images"
 	app.OnUsageError = usageErrorHandler
+	app.CommandNotFound = commandNotFoundHandler
 
 	app.Version = version.Version