libceph: report socket read/write error message

We need to set error_msg to something useful before calling ceph_fault();
do so here for try_{read,write}().  This is more informative than

libceph: osd0 192.168.106.220:6801 (null)

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>

net/ceph/messenger.c

@@ -2287,14 +2287,18 @@ restart:
 	ret = try_read(con);
 	if (ret == -EAGAIN)
 		goto restart;
-	if (ret < 0)
+	if (ret < 0) {
+		con->error_msg = "socket error on read";
 		goto fault;
+	}
 
 	ret = try_write(con);
 	if (ret == -EAGAIN)
 		goto restart;
-	if (ret < 0)
+	if (ret < 0) {
+		con->error_msg = "socket error on write";
 		goto fault;
+	}
 
 done:
 	mutex_unlock(&con->mutex);