Use enum instead of boolean for TMCH CA mode

Also more narrowly scopes a catch block in TmchCertificateAuthority.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144744847

javatests/google/registry/flows/EppCommitLogsTest.java

@@ -25,6 +25,7 @@ import static org.joda.time.DateTimeZone.UTC;
 import static org.joda.time.Duration.standardDays;
 
 import com.googlecode.objectify.Key;
+import google.registry.config.RegistryConfig.ConfigModule.TmchCaMode;
 import google.registry.flows.EppTestComponent.FakesAndMocksModule;
 import google.registry.model.domain.DomainResource;
 import google.registry.model.ofy.Ofy;
@@ -71,7 +72,7 @@ public class EppCommitLogsTest extends ShardableTestCase {
     SessionMetadata sessionMetadata = new HttpSessionMetadata(new FakeHttpSession());
     sessionMetadata.setClientId("TheRegistrar");
     DaggerEppTestComponent.builder()
-        .fakesAndMocksModule(new FakesAndMocksModule(clock, true))
+        .fakesAndMocksModule(new FakesAndMocksModule(clock, TmchCaMode.PILOT))
         .build()
         .startRequest()
         .flowComponentBuilder()