Error page: Use descriptive code for alloptions (#1379)

When the alloptions size exceeds the defined limit (1MB), we displayed an error page with a `1024` code. We can make that code more human-friendly so it's easier to identify the actual problem.

Fixes #1210

errors/alloptions-limit.html

@@ -146,7 +146,7 @@
 				<h1>Site Unavailable</h1>
 				<p>This site is temporarily unavailable, please try again in a few moments.</p>
 				<footer>
-					<span class="error">Error 1024</span>
+					<span class="error">Error 1024 (<code>alloptions</code>)</span>
 				</footer>
 			</div>
 		</div>