Add possibility to do custom action on the option parsing exceptions

[IvanPizhenko]

When I run my application with unknown option, like --nonsense, I get error Error: no such option: "--nonsense", and that's ok:

$ java -jar myapp.jar --nonsense
Error: no such option: "--nonsense"

But I'd like to print help message after that, but it seems like clikt doesn't do this, and there is no way to do it manually.

I've found this code:

    /**
     * Parse the command line and print helpful output if any errors occur.
     *
     * This function calls [parse] and catches and [CliktError]s that are thrown. Other errors are allowed to
     * pass through.
     */
    fun main(argv: List<String>) {
        try {
            parse(argv)
        } catch (e: ProgramResult) {
            exitProcessMpp(e.statusCode)
        } catch (e: PrintHelpMessage) {
            echo(e.command.getFormattedHelp())
            exitProcessMpp(if (e.error) 1 else 0)
        } catch (e: PrintCompletionMessage) {
            val s = if (e.forceUnixLineEndings) "\n" else currentContext.console.lineSeparator
            echo(e.message, lineSeparator = s)
            exitProcessMpp(0)
        } catch (e: PrintMessage) {
            echo(e.message)
            exitProcessMpp(if (e.error) 1 else 0)
        } catch (e: UsageError) {
            echo(e.helpMessage(), err = true)
            exitProcessMpp(e.statusCode)
        } catch (e: CliktError) {
            echo(e.message, err = true)
            exitProcessMpp(1)
        } catch (e: Abort) {
            echo(currentContext.localization.aborted(), err = true)
            exitProcessMpp(if (e.error) 1 else 0)
        }
    }

which prints out exception and exits program right away. I'd like there to be customization point:

        } catch (e: UsageError) {
            echo(e.helpMessage(), err = true)
            onUsageError(e) // <<< ADD THIS
            exitProcessMpp(e.statusCode)
        } catch (e: CliktError) {

// later on, something like this
  protected open fun onUsageError(e: UsageError) {
      // no action right here, customization point
  }

I even can try to implement this and send PR.