Render contest validation errors more nicely. Fixes #2891

Author: meisterT

webapp/src/Service/CheckConfigService.php

@@ -479,9 +479,15 @@ public function checkContestsValidate(): ConfigCheckItem
 
         $desc = '';
         foreach ($contesterrors as $cid => $errors) {
-            $desc .= "Contest: c$cid: " .
-                /* @phpstan-ignore-next-line */
-                    (count($errors) == 0 ? 'no errors' : (string)$errors) ."\n" .$cperrors[$cid];
+            $desc .= "Contest: c$cid: ";
+            if (count($errors) == 0) {
+                $desc .= "no errors\n";
+            } else {
+                $desc .= "errors:\n";
+                foreach ($errors as $error) {
+                    $desc .= "  - " . $error->getPropertyPath() . ": " . $error->getMessage() . "\n";
+                }
+            }
         }
 
         $this->stopwatch->stop(__FUNCTION__);