In the event of an error or an exception, when in production mode a custom error message is displayed:
Whoops! An error has occurred.
When in development mode a feature rich error page is show with a full stack trace of the route of the error.
The actual error is recorded in storage/logs/error.log. Any errors will be recorded in that file.
To change the development mode edit app/Config/app.php
'debug' => true, // When enabled the actual PHP errors will be shown.
By default it's set to true. When going to production turn this to false.