cPanel allows to easily create custom error pages for commonly HTTP errors, such as "404 Not Found" and "500 Internal Server Error" (and many others, much less common).

To do it, after the login to main cPanel page, scroll down to the "Advanced" section, and click on "Error Pages"


 

This will take you to the main error pages page, which lists many kinds of error pages that you can customize.

Let's select one, and click on the link: this will take you to an editing page, where you create your custom error page.
You can include one or more of the listed tags to further customize the page.
You can also use HTML tags in your page, making it easy to match the look of your error page to the rest of your web site.

You can customize any of the listed error pages, but you not forced to do it: there are preset standard error pages, which bell be used even if you don't personalize them.

The full list of customizable error pages:

400 (Bad request)
401 (Authorization required)
402 (Payment required)
403 (Forbidden)
404 (Not found)
405 (Method not allowed)
406 (Not acceptable)
407 (Proxy authentication required)
408 (Request timeout)
409 (Conflict)
410 (Gone)
411 (Length required)
412 (Precondition failed)
413 (Request entity too large)
414 (Request URI too large)
415 (Unsupported media type)
416 (Request range not satisfiable)
417 (Expectation failed)
422 (Unprocessable entity)
423 (Locked)
424 (Failed dependency)
500 (Internal server error)
501 (Not Implemented)
502 (Bad gateway)
503 (Service unavailable)
504 (Gateway timeout)
505 (HTTP version not supported)
506 (Variant also negotiates)
507 (Insufficient storage)
510 (Not extended)