freetdm: add deprecate warning to get last error stuff that nobody sets anyways

libs/freetdm/TODO

@@ -5,5 +5,9 @@
   then ftdm_event_t would be renamed to ftdm_oob_event_t and the enum_id renamed to type, then ftdm_span_next_event()
   will only return OOB events
 
-- query span hw status (connected/disconnected) on startup 
+- Deprecate last_error members. 
+  It requires a lot of discipline to set the last_error string for every failure.
+  It does not add much value to the user either, most of the errors are criptic and
+  cannot be shown to end users, we already provide extensive logging for problem
+  troubleshooting.
 

libs/freetdm/src/include/freetdm.h

@@ -1064,6 +1064,9 @@ FT_DECLARE(ftdm_codec_t) ftdm_channel_get_codec(const ftdm_channel_t *ftdmchan);
 /*! 
  * \brief Get the last error string for the channel
  *
+ * \deprecated This API will disappear in the future and not every 
+ *             FreeTDM API set the last error value
+ *
  * \param ftdmchan The channel to get the error from
  *
  * \retval The error string (not thread-safe, the string is per channel, not per thread)
@@ -1136,6 +1139,9 @@ FT_DECLARE(ftdm_status_t) ftdm_span_find(uint32_t id, ftdm_span_t **span);
 /*! 
  * \brief Get the last error string for the given span
  *
+ * \deprecated This API will disappear in the future and not every 
+ *             FreeTDM API set the last error value
+ *
  * \param span The span to get the last error from
  *
  * \retval character string for the last error