Website: improve error docs page for contributors

If quick-lint-js' wasm thingy is out of date, visiting the error
documentation page of the website can fail with a 500 internal server
error. Make the error message prettier and include information on how to
likely resolve the error.

This commit should hopefully not affect the public website.

Author: strager

website/public/errors/index.ejs.html

@@ -10,6 +10,7 @@
       let moduleCacheBust = "#" + Date.now();
 
       let {
+        ProblemsError,
         documentationDirectoryPath,
         loadErrorDocumentationFilesAsync,
         reportProblemsInDocumentsAsync,
@@ -20,7 +21,6 @@
       let documents = await loadErrorDocumentationFilesAsync(
         documentationDirectoryPath
       );
-      await reportProblemsInDocumentsAsync(documents);
       //%>
     </script>
     <title>quick-lint-js: errors and warnings</title>
@@ -75,6 +75,21 @@ <h1><a href="../">quick-lint-js</a></h1>
         quick-lint-js can find the following warnings and errors in your code:
       </p>
 
+      <% try { await reportProblemsInDocumentsAsync(documents); } catch (e) { if
+      (e instanceof ProblemsError) { %>
+      <div
+        style="background-color: #fdd; border: 1px solid #f00; padding: 0.5em"
+      >
+        <p>
+          quick-lint-js found inconsistencies in the error documentation. If
+          you're hacking on the website, rebuild quick-lint-js for the website
+          by following <code>wasm/README.md</code> and
+          <code>website/README.md</code>.
+        </p>
+        <p style="white-space: pre-line"><%= e %></p>
+      </div>
+      <% } else { throw e; } } %>
+
       <ul class="table-of-contents">
         <% for (let doc of documents) { %>
         <li>

website/src/error-documentation.mjs

@@ -239,7 +239,7 @@ export async function reportProblemsInDocumentsAsync(documents) {
     foundProblems.push(...(await doc.findProblemsAsync()));
   }
   if (foundProblems.length !== 0) {
-    throw new Error(
+    throw new ProblemsError(
       `found problems in error documents:\n${foundProblems.join("\n")}`
     );
   }
@@ -266,6 +266,8 @@ export async function loadErrorDocumentationFilesAsync(rootPath) {
   return documents;
 }
 
+export class ProblemsError extends Error {}
+
 // quick-lint-js finds bugs in JavaScript programs.
 // Copyright (C) 2020  Matthew "strager" Glazar
 //