Put this in your Laravel's Handler.php to format all the exceptions to JSON:API format

Handler.php

<?php

namespace App\Exceptions;

use Exception;
use Illuminate\Foundation\Exceptions\Handler as ExceptionHandler;
use Illuminate\Http\Exceptions\HttpResponseException;
use Illuminate\Auth\AuthenticationException;
use Illuminate\Validation\ValidationException;
use Illuminate\Auth\Access\AuthorizationException;
use Illuminate\Support\Facades\Response;

class Handler extends ExceptionHandler
{
    /**
     * A list of the exception types that are not reported.
     *
     * @var array
     */
    protected $dontReport = [
        //
    ];

    /**
     * A list of the inputs that are never flashed for validation exceptions.
     *
     * @var array
     */
    protected $dontFlash = [
        'password',
        'password_hash',
        'password_confirmation',
        'auth_key',
    ];

    /**
     * Report or log an exception.
     *
     * @param  \Exception  $exception
     * @return void
     */
    public function report(Exception $exception)
    {
        parent::report($exception);
    }

    /**
     * Render an exception into an HTTP response.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Exception  $exception
     * @return \Illuminate\Http\Response
     */
    public function render($request, Exception $exception)
    {
        if ($request->expectsJson()) {
            return $this->handleApiException($request, $exception);
        }

        if ($exception instanceof AuthorizationException) {
            return $this->handleAuthorization();
        }

        return parent::render($request, $exception);
    }

    private function handleAuthorization()
    {
        return Response::redirectGuest('identify')
            ->withErrors([
                'login' => __(\Auth::check() ? 'errors.401' : 'auth.unauthorised')
            ]);
    }

    private function handleApiException($request, Exception $exception)
    {
        $exception = $this->prepareException($exception);

        if ($exception instanceof HttpResponseException) {
            $exception = $exception->getResponse();
        }

        if ($exception instanceof AuthenticationException) {
            $exception = $this->unauthenticated($request, $exception);
        }

        if ($exception instanceof ValidationException) {
            $exception = $this->convertValidationExceptionToResponse($exception, $request);
        }

        return $this->customApiResponse($exception);
    }

    private function customApiResponse($exception)
    {
        $response = [];
        $response['status'] = (method_exists($exception, 'getStatusCode'))
            ? $exception->getStatusCode()
            : 500;

        if (! method_exists($exception, 'getMessage')) {
            if ($response['status'] !== 422) {
                $response['message'] = __('errors.' . $response['status']);
            } else {
                $response['message'] = $exception->original['message'];
                $response['errors'] = $exception->original['errors'];
            }
        } else {
            $response['message'] = $exception->getMessage();
        }

        if (config('app.debug') && $response['status'] === 500) {
            // TODO: Trace as concatenated string on message?
            $response['trace'] = $exception->getTrace();
            $response['code'] = $exception->getCode();
        }

        return response()->json($response, $response['status']);
    }
}