Allow enabling of dummy error_strerror() to support some use-cases

Enable a dummy error function to make use of error_strerror() in
third party libraries easier.

Disable if you run into name conflicts and want to really remove the
error_strerror()
Fixed timing difference resulting from badly formatted padding.
Disable debug messages that can introduce a timing side channel.

Introduced the POLARSSL_SSL_DEBUG_ALL flag to enable all these debug
messages in case somebody does want to see the reason checks fail.
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.
Updated for PolarSSL 1.2.5