Bugfix
   * Failures of alternative implementations of AES or DES single-block
     functions enabled with MBEDTLS_AES_ENCRYPT_ALT, MBEDTLS_AES_DECRYPT_ALT,
     MBEDTLS_DES_CRYPT_ECB_ALT or MBEDTLS_DES3_CRYPT_ECB_ALT were ignored.
     This does not concern the implementation provided with Mbed TLS,
     where this function cannot fail, or full-module replacements with
     MBEDTLS_AES_ALT or MBEDTLS_DES_ALT. Reported by Armelle Duboc in #1092.
   * Some failures of HMAC operations were ignored. These failures could only
     happen with an alternative implementation of the underlying hash module.

Features
   * Warn if errors from certain functions are ignored. This is currently
     supported on GCC-like compilers and on MSVC and can be configured through
     the macro MBEDTLS_CHECK_RETURN. The warnings are always enabled
     (where supported) for critical functions where ignoring the return
     value is almost always a bug. Enable the new configuration option
     MBEDTLS_CHECK_RETURN_WARNING to get warnings for other functions. This
     is currently implemented in the AES, DES and md modules, and will be
     extended to other modules in the future.