Clear, fail-fast error messages

Distinguish between programmer errors and runtime/user-facing behavior changes when deciding how to handle failures, and make error messages easy to localize and act on.

• For invalid or misused API calls (e.g. bad argument syntax, duplicate command registration, invalid configuration), fail fast by throwing immediately with a clear, specific message rather than silently ignoring the problem or corrupting internal state. Example: throwing passThroughOptions cannot be used for 'sub' without turning on enablePositionalOptions for parent command(s) instead of silently continuing, or throwing when the same subcommand name is registered twice instead of letting it silently overwrite internals.
• For changes that affect existing end-user runtime behavior (not pure programmer misuse), prefer backward-compatible handling such as logging a warning or keeping old behavior rather than introducing a hard break, especially when the impact is ambiguous (e.g. detecting both action and command:* listeners registered, or detecting incompatible use of .parse() with async handlers). Only throw hard errors once the new behavior is well understood and justified.
• Build error messages compositionally instead of hardcoding natural-language words mixed with dynamic values, so messages remain translatable and consistent. Avoid baking words like “option” or “environment variable” directly into message templates; instead construct the full phrase (e.g. “option ‘–wrong’” or “’–wrong’ (required environment variable: RIGHT)”) as a single substitutable unit.
• Keep thrown messages focused: include only the context actually needed to diagnose and fix the issue (e.g. the failing command/option name), and avoid adding extra context that doesn’t help resolution.

Example:

// Fail fast on programmer misuse
if (parent && command._passThroughOptions && !parent._enablePositionalOptions) {
  throw new Error(`passThroughOptions cannot be used for '${command._name}' without turning on enablePositionalOptions for parent command(s)`);
}

// Prefer warning over breaking change for ambiguous behavior changes
if (isThenable(result) && !this._asyncParsing) {
  console.warn('Warning: .parse() used with an async handler; consider .parseAsync()');
}