Check registrar existence prior to verifying access

This way the error messages are more sensible when a registrar doesn't exist
(which realistically shouldn't happen in the typical case anyway).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=240376239

javatests/google/registry/request/auth/AuthenticatedRegistrarAccessorTest.java

@@ -349,7 +349,7 @@ public class AuthenticatedRegistrarAccessorTest {
     expectGetRegistrarFailure(
         "BadClientId",
         GAE_ADMIN,
-        "admin admin@gmail.com doesn't have access to registrar BadClientId");
+        "Registrar BadClientId does not exist");
     verifyZeroInteractions(lazyGroupsConnection);
   }