Support /a/ for plugin REST APIs

Without the /a/, users are unable to script REST API calls using tools
such as curl, as the CurrentUser wasn't being provided.

Also fix a minor bug when anonymous users tried to access /a/ URLs.
We were throwing an exception when building up our error message.

Change-Id: I5ce870960aa107001c4014fefcf914adc366cd8a
Signed-off-by: Brad Larson <bklarson@gmail.com>

gerrit-httpd/src/main/java/com/google/gerrit/httpd/RestApiServlet.java

@@ -102,10 +102,13 @@ public abstract class RestApiServlet extends HttpServlet {
       CapabilityControl ctl = user.getCapabilities();
       if (!ctl.canPerform(rc.value()) && !ctl.canAdministrateServer()) {
         String msg = String.format(
-            "fatal: %s does not have \"%s\" capability.",
-            Objects.firstNonNull(user.getUserName(),
-                ((IdentifiedUser)user).getNameEmail()),
-            rc.value());
+          "fatal: %s does not have \"%s\" capability.",
+          Objects.firstNonNull(
+            user.getUserName(),
+            user instanceof IdentifiedUser
+              ? ((IdentifiedUser) user).getNameEmail()
+              : user.toString()),
+          rc.value());
         throw new RequireCapabilityException(msg);
       }
     }

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/HttpPluginModule.java

@@ -28,6 +28,7 @@ public class HttpPluginModule extends ServletModule {
   protected void configureServlets() {
     bind(HttpPluginServlet.class);
     serve("/plugins/*").with(HttpPluginServlet.class);
+    serveRegex("^/(?:a/)?plugins/(.*)?$").with(HttpPluginServlet.class);
 
     bind(StartPluginListener.class)
       .annotatedWith(UniqueAnnotations.create())