freetdm: add debugging msgs for zt wait errors

libs/freetdm/mod_freetdm/mod_freetdm.c

@@ -657,7 +657,7 @@ static switch_status_t channel_read_frame(switch_core_session_t *session, switch
 	status = ftdm_channel_wait(tech_pvt->ftdmchan, &wflags, chunk);
 	
 	if (status == FTDM_FAIL) {
-		ftdm_log(FTDM_LOG_WARNING, "failed to wait for I/O\n");
+		ftdm_log(FTDM_LOG_ERROR, "Failed to wait for I/O\n");
 		goto fail;
 	}
 	

libs/freetdm/src/ftmod/ftmod_zt/ftmod_zt.c

@@ -889,6 +889,7 @@ static FIO_WAIT_FUNCTION(zt_wait)
 	*flags = 0;
 
 	if (pfds[0].revents & POLLERR) {
+		ftdm_log_chan_msg(ftdmchan, FTDM_LOG_ERROR, "DAHDI device got POLLERR\n");
 		result = -1;
 	}
 
@@ -900,6 +901,7 @@ static FIO_WAIT_FUNCTION(zt_wait)
 
 	if (result < 0){
 		snprintf(ftdmchan->last_error, sizeof(ftdmchan->last_error), "Poll failed");
+		ftdm_log_chan(ftdmchan, FTDM_LOG_ERROR, "Failed to poll DAHDI device: %s\n", strerror(errno));
 		return FTDM_FAIL;
 	}