ReviewersResolver: Log failure to list accounts for group as error

Increase the log level from WARN to ERROR.

Change-Id: I9960eeb835c1a3167572dcd7c2e7a4e045dedf12
diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersResolver.java b/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersResolver.java
index a1f9d9a..af84789 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersResolver.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersResolver.java
@@ -162,7 +162,7 @@
           project,
           group);
     } catch (NoSuchProjectException | IOException | OrmException e) {
-      log.warn(
+      log.error(
           "For the change {} of project {}: failed to list accounts for group {}.",
           changeNumber,
           project,