throw exception in nice_die during tests

Exits during testing will break the whole test suite. It makes more
sense to throw an exception in that case. The exception's stack trace
will help to debug the actual problem

inc/init.php

@@ -527,6 +527,9 @@ function nice_die($msg){
 </body>
 </html>
 EOT;
+    if(defined('DOKU_UNITTEST')) {
+        throw new RuntimeException('nice_die: '.$msg);
+    }
     exit(1);
 }