wolfSSL error codes are mostly positive numbers, but esp-tls potentially
non-block API (read/write) returns ssize_t, i.e. bytes read/written if
>0, errorcode otherwise. To comply with this API we have to conditionate
the wolfssl return codes to negative numbers, preferably the same codes
as mbedTLS codes.
Transport's error_handle is used to capture different types of errors
including esp_err_t from esp-tls and socket/system errors.
This change adds the following error codes for TCP transport:
* connection closed by a FIN flag (clean closure from server)
* DNS resolution problem
* connection timeout
These errors are already defined in esp-tls component and since this
component will be used in the future for both TCP and SSL transport, we
currently report these issues in transport::error_handle::esp_tls_last_error
of standard esp error type (esp_err_t)
Closes https://github.com/espressif/esp-mqtt/issues/182
Aditya Patwardhan