got rid of all uses of strerror

This required some minor refactoring of other things and there were some
other small cleanups I noticed along the way.
diff --git a/aos/common/network/send_socket.cc b/aos/common/network/send_socket.cc
index 03bc57c..9886893 100644
--- a/aos/common/network/send_socket.cc
+++ b/aos/common/network/send_socket.cc
@@ -20,8 +20,7 @@
   }
 
   if (connect(socket_, &addr_.addr, sizeof(addr_)) < 0) {
-    LOG(ERROR, "couldn't connect to ip '%s' because of %d: %s\n", robot_ip,
-        errno, strerror(errno));
+    PLOG(ERROR, "couldn't connect to ip '%s'", robot_ip);
     return last_ret_ = 1;
   }