Implement retry for transient errors in WHOIS server

We now attempt to retry Whois queries in the event of a short-lived error. Currently, we consider 'DatastoreTimeoutException' and 'DatastoreFailureException' as transient.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152044934

java/google/registry/whois/WhoisException.java

@@ -68,4 +68,11 @@ public final class WhoisException extends Exception implements WhoisResponse {
         .toString();
     return WhoisResponseResults.create(plaintext, 0);
   }
+
+  /** Exception that wraps WhoisExceptions returned from Retrier. */
+  public static final class UncheckedWhoisException extends RuntimeException {
+    UncheckedWhoisException(WhoisException whoisException) {
+      super(whoisException);
+    }
+  }
 }