Catch UnprocessableEntityException

Currently not caught when creating a branch.
From 3.7 it is propagated to the client.

Change-Id: I85ea421c24dafa88809ebaad86e1541332957769

src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java b/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
index 704ae5c..d05eeaa 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
@@ -33,6 +33,7 @@
 import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.IdString;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
 import com.google.gerrit.server.config.PluginConfigFactory;
 import com.google.gerrit.server.data.AccountAttribute;
 import com.google.gerrit.server.data.RefUpdateAttribute;
@@ -170,7 +171,8 @@
               | ResourceConflictException
               | IOException
               | PermissionBackendException
-              | NoSuchProjectException e) {
+              | NoSuchProjectException
+              | UnprocessableEntityException e) {
             log.error("Failed to create backup ref {}", backupRef, e);
           }
         }