Handle non existent commands during development

[xLuxy]

Currently, when freestanding is set to true and you try to use a command which is not registered the bot will throw an exception. (I've used the modlogs command for testing)

This PR fixes that by adding a missing return statement.

As a small improvement to this, i've also added a response to whenever a command has not been registered which can help during development.

Context:

🛑 [2026.03.17 20:18:25.763] [error] TypeError: Cannot read properties of undefined (reading 'options')
    at CommandHandler.handle_slash_comand (file:///D:/dev/wheatley/build/src/command-handler.js:157:28)
    at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
    at async CommandHandler.on_interaction (file:///D:/dev/wheatley/build/src/command-handler.js:331:17)
Error
  TypeError: Cannot read properties of undefined (reading 'options')
      at CommandHandler.handle_slash_comand (file:///D:/dev/wheatley/build/src/command-handler.js:157:28)
      at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
      at async CommandHandler.on_interaction (file:///D:/dev/wheatley/build/src/command-handler.js:331:17)
  Trace
      at M.error (file:///D:/dev/wheatley/build/src/utils/debugging-and-logging.js:61:17)
      at Wheatley.critical_error (file:///D:/dev/wheatley/build/src/wheatley.js:229:15)
      at CommandHandler.on_interaction (file:///D:/dev/wheatley/build/src/command-handler.js:405:31)
      at process.processTicksAndRejections (node:internal/process/task_queues:104:5)

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Rinzii]

Iirc #210 partially already handles this, but I'd say this should expand on what I did in that PR well so PR is still valid.