Added sending of alert messages in case of decryption failures as per RFC

The flag POLARSSL_SSL_ALERT_MESSAGES switched between enabling and
disabling the sending of alert messages that give adversaries intel
about the result of their action. PolarSSL can still communicate with
other parties if they are disabled, but debugging of issues might be
harder.
diff --git a/ChangeLog b/ChangeLog
index c6f90a2..86a7727 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,9 @@
    * Allow enabling of dummy error_strerror() to support some use-cases
    * Debug messages about padding errors during SSL message decryption are
      disabled by default and can be enabled with POLARSSL_SSL_DEBUG_ALL 
+   * Sending of security-relevant alert messages that do not break
+     interoperability can be switched on/off with the flag
+	 POLARSSL_SSL_ALL_ALERT_MESSAGES
 
 Security
    * Removed timing differences during SSL message decryption in