Add assertTldsExist(Iterable<String>) to check multiple TLDs at once

This is better than calling assertTldExists() inside a for loop because you can throw a single exception reporting all bad TLDs at once rather than only getting as far as the first failure.  And then it's also a one-liner instead of 3 lines.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152412876

java/google/registry/tools/GenerateZoneFilesCommand.java

@@ -14,7 +14,7 @@
 
 package google.registry.tools;
 
-import static google.registry.model.registry.Registries.assertTldExists;
+import static google.registry.model.registry.Registries.assertTldsExist;
 import static org.joda.time.DateTimeZone.UTC;
 import static org.joda.time.Duration.standardMinutes;
 
@@ -54,9 +54,7 @@ final class GenerateZoneFilesCommand implements ServerSideCommand {
 
   @Override
   public void run() throws IOException {
-    for (String tld : mainParameters) {
-      assertTldExists(tld);
-    }
+    assertTldsExist(mainParameters);
     ImmutableMap<String, Object> params = ImmutableMap.of(
         "tlds", mainParameters,
         "exportTime", exportDate.toString());