Added middleware that mitigates big error traces being logged for those not using mercure

module/Rest/src/Middleware/Mercure/NotConfiguredMercureErrorHandler.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Rest\Middleware\Mercure;
+
+use Mezzio\ProblemDetails\ProblemDetailsResponseFactory;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
+use Psr\Log\LoggerInterface;
+use Shlinkio\Shlink\Rest\Exception\MercureException;
+
+class NotConfiguredMercureErrorHandler implements MiddlewareInterface
+{
+    public function __construct(private ProblemDetailsResponseFactory $respFactory, private LoggerInterface $logger)
+    {
+    }
+
+    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
+    {
+        try {
+            return $handler->handle($request);
+        } catch (MercureException $e) {
+            // Throwing this kind of exception makes a big error trace to be logged, for anyone who has decided to not
+            // use mercure.
+            // It happens every time the shlink-web-client is opened, so this mitigates the problem by just logging a
+            // simple warning, and casting the exception to a response on the fly.
+            $this->logger->warning($e->getMessage());
+            return $this->respFactory->createResponseFromThrowable($request, $e);
+        }
+    }
+}