ChangeEditModifier: Fix exception message when ref cannot be updated

RefUpdate doesn't implement toString and hence the exception message was
not very useful.

Change-Id: Icf24aa5e9955affd4a369c1c5faa074b8a1460fd
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/edit/ChangeEditModifier.java

@@ -566,7 +566,13 @@ public class ChangeEditModifier {
     try (RevWalk revWalk = new RevWalk(repository)) {
       RefUpdate.Result res = ru.update(revWalk);
       if (res != RefUpdate.Result.NEW && res != RefUpdate.Result.FORCED) {
-        throw new IOException("update failed: " + ru);
+        throw new IOException(
+            "cannot update "
+                + ru.getName()
+                + " in "
+                + repository.getDirectory()
+                + ": "
+                + ru.getResult());
       }
     }
   }