API changes
   * Remove SSL error codes `MBEDTLS_ERR_SSL_CERTIFICATE_REQUIRED`
     and `MBEDTLS_ERR_SSL_INVALID_VERIFY_HASH` which are never
     returned from the public SSL API.
   * Remove `MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE` and return
     `MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL` instead.