Return -sockerrno when conn_send() failed

Currently, conn_*_send returns -1 on failure.
Return more meaningful code to make debugging easier.

Author: mojyack

src/conn_mux.c

@@ -522,6 +522,7 @@ int conn_mux_send(juice_agent_t *agent, const addr_record_t *dst, const char *da
 
 	int ret = udp_sendto(registry_impl->sock, data, size, dst);
 	if (ret < 0) {
+		ret = -sockerrno;
 		if (sockerrno == SEAGAIN || sockerrno == SEWOULDBLOCK)
 			JLOG_INFO("Send failed, buffer is full");
 		else if (sockerrno == SEMSGSIZE)

src/conn_poll.c

@@ -412,6 +412,7 @@ int conn_poll_send(juice_agent_t *agent, const addr_record_t *dst, const char *d
 
 	int ret = udp_sendto(conn_impl->sock, data, size, dst);
 	if (ret < 0) {
+		ret = -sockerrno;
 		if (sockerrno == SEAGAIN || sockerrno == SEWOULDBLOCK)
 			JLOG_INFO("Send failed, buffer is full");
 		else if (sockerrno == SEMSGSIZE)

src/conn_thread.c

@@ -260,6 +260,7 @@ int conn_thread_send(juice_agent_t *agent, const addr_record_t *dst, const char
 
 	int ret = udp_sendto(conn_impl->sock, data, size, dst);
 	if (ret < 0) {
+		ret = -sockerrno;
 		if (sockerrno == SEAGAIN || sockerrno == SEWOULDBLOCK)
 			JLOG_INFO("Send failed, buffer is full");
 		else if (sockerrno == SEMSGSIZE)