Throw for author errors and improve

[shadowspawn]

Pull Request

Problem

Inconsistent handling of author errors (log vs throw).
Accidental executable subcommand is a common error.
Problems with variadic argument are detected when used, not earlier when added.

Solution

• use throw for author errors
• detect misplaced variadic argument as added, rather than later when used
• add tips to missing executable error

Example of the extended error message for missing executable subcommand:

const program = require("commander");
program
  .command('sub', 'description')
  .action(() => { 'Intended action handler'});
program.parse(process.argv);

% node missing.js sub
/Users/john/Documents/Sandpits/commander/my-fork/index.js:746
        throw new Error(executableMissing);
        ^

Error: 'missing-sub' does not exist
 - if 'sub' is not meant to be an executable command, remove description parameter from '.command()' and use '.description()' instead
 - if the default executable name is not suitable, use the executableFile option to supply a custom name
    at ChildProcess.<anonymous> (/Users/john/Documents/Sandpits/commander/my-fork/index.js:746:15)
    at ChildProcess.emit (events.js:321:20)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:273:12)
    at onErrorNT (internal/child_process.js:467:16)
    at processTicksAndRejections (internal/process/task_queues.js:84:21)

[abetomo]

Great!