Fixing "Error handling difficult and not reliable #38"

[noneymous]

Here comes the implementation of my suggestion to fix #38. Please have a look and see how this works for you. I tested it against some scan targets and am still running it against even more. You are probably using different Nmap arguments than me, so I'm curious about your experiences.

Result processing would be like:

1. Check if error is returned (includes package errors and nmap errors). If error is returned, something went most probably wrong and you can abort. Warning's might give you more details/flexibility. Most returned errors are defined as error constants in errors.go. Have a look there to see what issues might come back to handle them individually
2. OptionA: Check warnings for things that might be an issue for you and abort or proceed
   OptionB: Ignore warnings at all and just work with the returned XML. It already contains a lot of details

Here is sample usage code:

// Prepare Nmap scan
proc, err := nmap.NewScanner(options...)
if err != nil {
    fmt.Println("Could not initialize Nmap")
    return
}

// Execute Nmap scan
result, warnings, err := proc.Run()

// Check for nmap errors and optionally handle them individually
if err != nil {
	switch err {
		case nmap.ErrScanTimeout:
        	fmt.Println("ERROR: Nmap scan ran into timeout")
    	case nmap.ErrParseOutput:
        	fmt.Printf("ERROR: Nmap output could not be parsed: %w\n", warnings[len(warnings)-1])
    	case nmap.ErrExcludeList:
        	fmt.Println("ERROR: Nmap could not resolve host(s) on exclude list")
    	default:
        	fmt.Printf("ERROR: Nmap scan failed with unexpected error: %w\n", err)
	}
	return
}

// Check for nmap warnings that might be relevant for you
for _, warning := range warnings {
    switch {
 		case strings.Contains(warning, "Failed to resolve"):
        	fmt.Println("A target could not be resolved.") 
    	case strings.Contains(warning, "No targets were specified")
			fmt.Println("The only target is blacklisted.")
    	case strings.Contains(warning, "TTVAR has grown")
        	fmt.Println("Just some TTVAR change notification.")
    	default:
        	fmt.Println(fmt.Sprintf("Other warning: %s", warning))
    }
}

// Process result
fmt.Println(result)

[Ullaakut]

Thank you very much for this contribution @noneymous! This is great.

Looks solid so far! I have a suggestion that impacts a few little things, and also it will require unit tests before it can be merged. I didn't test it yet but I will as soon as I have some time 🙏

[noneymous]

Good idea, I introduced named return values and it's already commited.

Just the warning unit test outstanding. Maybe tomorrow.

[noneymous]

Sorry, I'm in the wrong environment to get the tests running. I feel this would take me too much time but might be a simple thing for you...

[TerminalFi]

Awaiting Unit Tests

[TerminalFi]

Looks promising and well laid out. I agree, we need unit tests as this is a pretty big change across many files.

[Ullaakut]

I'll test it this weekend :) It's on my calendar now:

[Ullaakut]

Looks good to me

[noneymous]

Thank you guys

[noneymous]

What's the plan, when do you intend to merge?

[TerminalFi]

@noneymous we will merge once tests have been completed. We thank you for your patience. Both myself and @Ullaakut have busy schedules at times.

[Ullaakut]

I'll wait for @TheSecEng to test it as well and once it's fine for him, we can merge the PR :)

[TerminalFi]

Cleared.