Check return value of connect to trap sooner (#19)

Check the return value of connect and error if it did not succeed.
This make socket errors more obvious since the TLS handshake has not
started at this point.

Author: c1728p9

tls-client/main.cpp

@@ -207,7 +207,12 @@ class HelloHTTPS {
 
         /* Connect to the server */
         mbedtls_printf("Connecting with %s\r\n", _domain);
-        _tcpsocket->connect( _domain, _port );
+        ret = _tcpsocket->connect(_domain, _port);
+        if (ret != NSAPI_ERROR_OK) {
+            mbedtls_printf("Failed to connect\r\n");
+            onError(_tcpsocket, -1);
+            return;
+        }
 
        /* Start the handshake, the rest will be done in onReceive() */
         mbedtls_printf("Starting the TLS handshake...\r\n");