Fix doc index generation to include `error` rules.

tool/doc.dart

Index: tool/doc.dart
diff --git a/tool/doc.dart b/tool/doc.dart
index d1064626e96663d6ae9e663a804d876ae73fbc46..8ef786f1b1520600e72276349d6e2531a52dfdb9 100644
--- a/tool/doc.dart
+++ b/tool/doc.dart
@@ -55,8 +55,8 @@ const ruleLeadMatter = 'Rules are organized into familiar rule groups.';
 final List<LintRule> rules =
     new List<LintRule>.from(ruleRegistry, growable: false)..sort();
 
-String get enumeratePubRules => rules
-    .where((r) => r.group == Group.pub)
+String get enumerateErrorRules => rules
+    .where((r) => r.group == Group.errors)
     .map((r) => '${toDescription(r)}')
     .join('\n\n');
 
@@ -65,6 +65,11 @@ String get enumerateGroups => Group.builtin
         '<li><strong>${g.name}</strong> - ${markdownToHtml(g.description)}</li>')
     .join('\n');
 
+String get enumeratePubRules => rules
+    .where((r) => r.group == Group.pub)
+    .map((r) => '${toDescription(r)}')
+    .join('\n\n');
+
 String get enumerateStyleRules => rules
     .where((r) => r.group == Group.style)
     .map((r) => '${toDescription(r)}')
@@ -212,6 +217,10 @@ class Indexer {
                ${markdownToHtml(ruleFootMatter)}
             </p>
 
+            <h2 id="styleguide-rules">Error Rules</h2>
+
+               $enumerateErrorRules
+
             <h2 id="styleguide-rules">Style Rules</h2>
 
                $enumerateStyleRules