Don't leak IOException error messages from CommitValidators

These may leak low-level implementation details that are useless to
end users. Log instead.

Change-Id: I86e81d2b9f0c3dcef7f88f234bb8eef83e582595
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/validators/CommitValidators.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/validators/CommitValidators.java
index 0c0065c..cad5174 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/validators/CommitValidators.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/validators/CommitValidators.java
@@ -538,7 +538,9 @@
         }
         return Collections.emptyList();
       } catch (IOException e) {
-        throw new CommitValidationException(e.getMessage(), e);
+        String m = "error checking banned commits";
+        log.warn(m, e);
+        throw new CommitValidationException(m, e);
       }
     }
   }