", e" added like for the other "Log.warn"

So the full stack trace can be displayed. I also fixed the typo (Unkown -> Unknown).

src/java/org/jivesoftware/openfire/keystore/TrustStore.java

@@ -213,7 +213,7 @@ public class TrustStore extends CertificateStore
         }
         catch ( Exception e )
         {
-            Log.warn( "Unkown exception while validating certificate chain:" + e.getMessage() );
+            Log.warn( "Unknown exception while validating certificate chain:", e );
         }
         return null;
     }