fix unhandled error leading to the panic

In the case when connection to the NGS is refused.

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1f590b4]

goroutine 1 [running]:
github.com/mantil-io/mantil/cli/controller/invoke.(*LambdaClient).Do(0xc000663a10, {0x2403a72, 0x6}, {0x218bf40, 0xc000438000}, {0x218bf80, 0xc000afc020})
	github.com/mantil-io/mantil/cli/controller/invoke/invoke.go:239 +0x134
github.com/mantil-io/mantil/cli/controller.(*Setup).create(0xc000414f00, 0xc00020cc40)
	github.com/mantil-io/mantil/cli/controller/setup.go:133 +0x3c8
github.com/mantil-io/mantil/cli/controller.(*Setup).Create(0xc000414f00, 0xc0005a5c20)
	github.com/mantil-io/mantil/cli/controller/setup.go:91 +0x325
github.com/mantil-io/mantil/cli/cmd.newAwsInstallCommand.func1(0xc0003bd400, {0xc0003a27c0, 0x2, 0x2})
	github.com/mantil-io/mantil/cli/cmd/commands.go:80 +0x1a7
github.com/spf13/cobra.(*Command).execute(0xc0003bd400, {0xc0003a27a0, 0x2, 0x2})
	github.com/spf13/[email protected]/command.go:856 +0x60e
github.com/spf13/cobra.(*Command).ExecuteC(0xc000377680)
	github.com/spf13/[email protected]/command.go:974 +0x3bc
github.com/mantil-io/mantil/cli/cmd.Execute()
	github.com/mantil-io/mantil/cli/cmd/root.go:24 +0x3e
main.run()
	github.com/mantil-io/mantil/cli/main.go:75 +0x275
main.main()
	github.com/mantil-io/mantil/cli/main.go:27 +0x2e
```

cli/controller/invoke/invoke.go

@@ -219,6 +219,9 @@ func Lambda(invoker Invoker, functionName string, logSink LogSinkCallback) *Lamb
 
 func (l *LambdaClient) Do(method string, req, rsp interface{}) error {
 	lsn, err := newListener(nil, rsp, l.logSink)
+	if err != nil {
+		return err
+	}
 
 	var payload []byte
 	if req != nil {