Add a warning log on BlockedKeyword validation exceptions

I initially planned to upgrade ComitValidators log in
change 337614, but it turned out infeasible due to semantics of the method.

We are missing missing debug logs on merge failures, multiple times observed with this plugins.

Change-Id: Ifc686df6855f9f256d0372bd429b89f9f7346c66
diff --git a/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java b/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java
index 7e6b9d6..737796b 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java
@@ -174,6 +174,7 @@
                   receiveEvent.revWalk,
                   blockedKeywordPatterns.values(),
                   cfg);
+
           if (!messages.isEmpty()) {
             throw new CommitValidationException(
                 "includes files containing blocked keywords", messages);
@@ -184,6 +185,7 @@
         | IOException
         | ExecutionException
         | DiffNotAvailableException e) {
+      logger.atWarning().withCause(e).log("blocked keywords commit validation failed");
       throw new CommitValidationException("failed to check on blocked keywords", e);
     }
     return Collections.emptyList();