chore(docs): improve error doc layout and linking

You can now link to an error by its name, namespace:name or error:namespace:name.
For example these would all link to https://docs.angularjs.org/error/$compile/ctreq

```
{@link ctreq}
{@link $compile:ctreq}
{@link error:$compile:ctreq}
```

Author: petebacondarwin

docs/config/index.js

@@ -125,10 +125,16 @@ module.exports = new Package('angularjs', [
   });
 
   computeIdsProcessor.idTemplates.push({
-    docTypes: ['error', 'errorNamespace'],
+    docTypes: ['error'],
+    getId: function(doc) { return 'error:' + doc.namespace + ':' + doc.name; },
+    getAliases: function(doc) { return [doc.name, doc.namespace + ':' + doc.name, doc.id]; }
+  },
+  {
+    docTypes: ['errorNamespace'],
     getId: function(doc) { return 'error:' + doc.name; },
     getAliases: function(doc) { return [doc.id]; }
-  });
+  }
+  );
 })
 
 .config(function(checkAnchorLinksProcessor) {

docs/config/templates/error.template.html

@@ -1,7 +1,7 @@
 {% extends "base.template.html" %}
 
 {% block content %}
-<h1>Error: {$ doc.id $}
+<h1>Error: {$ doc.namespace $}:{$ doc.name $}
   <div><span class='hint'>{$ doc.fullName $}</span></div>
 </h1>