fix: ensure $error is actually set, not just empty

The variable is undefined when debug is off, which means that this template itself results in an error.
Changed conditional to use `isset()` instead.

Signed-off-by: Matthew Weier O'Phinney <matthew@weierophinney.net>

Author: weierophinney

templates/error/error.phtml

@@ -9,7 +9,7 @@
         <a href="/chat">contact us via chat</a>.
     </p>
 
-    <?php if (! empty($error)) : ?>
+    <?php if (isset($error)) : ?>
     <pre><?= $this->e($error) ?></pre>
     <?php endif ?>
 </section>