Make Router reject classes with no @Action-returning methods

This provides a safeguard against using TypeInstantiator to resolve the component class, where if resolution is done incorrectly, you end up with java.lang.Object.  Formerly, that would have "succeeded" in generating a Router for Object, which of course has no methods that return @Action classes.  Such a router is pretty useless, so it's better to make Router stricter and have it fail if you give it such a class by accident.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148353224

java/google/registry/request/Router.java

@@ -14,6 +14,7 @@
 
 package google.registry.request;
 
+import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Throwables.throwIfUnchecked;
 
 import com.google.common.base.Function;
@@ -47,6 +48,8 @@ final class Router {
 
   private Router(Class<?> componentClass) {
     this.routes = extractRoutesFromComponent(componentClass);
+    checkArgument(
+        !this.routes.isEmpty(), "No routes found for class: %s", componentClass.getCanonicalName());
   }
 
   /** Returns the appropriate action route for a request. */