Make awesome-client exit with a meaningful value on error

As discussed on #awesome, the attached patch makes awesome-client exit
with a meaningful value (i.e. that of errno) when it encounters an
error. Since the most frequent error with awesome-client is a mismatch
in the socket path, there is an explicit case for ENOENT errors. I
thought of adding a matching fprintf in awesome.c, but you can tell what
socket awesome is listening on by looking at what ~/.awesome_ctl.* file
you have.

CAVEAT: Exiting on error may break setups such as:

	while true; do
		echo "some text"
	done | /path/to/awesome-client

which relied on awesome-client continuing to send to the given socket
(although failing) until EOF was encountered on stdin.

awesome-client.c

@@ -34,7 +34,7 @@ main(int argc, char **argv)
 {
     struct sockaddr_un *addr;
     char buf[1024];
-    int csfd;
+    int csfd, ret_value = EXIT_SUCCESS;
 
     csfd = get_client_socket();
     if(argc > 1)
@@ -46,12 +46,25 @@ main(int argc, char **argv)
         return EXIT_FAILURE;
 
     while(fgets(buf, sizeof(buf), stdin))
+    {
         if(sendto(csfd, buf, a_strlen(buf), MSG_NOSIGNAL,
                   (const struct sockaddr *) addr, sizeof(struct sockaddr_un)) == -1)
-            perror("error sending datagram");
+        {
+            switch (errno)
+            {
+                case ENOENT:
+                    fprintf(stderr, "Can't write to %s\n", addr->sun_path);
+                    break;
+                default:
+                    perror("error sending datagram");
+            }
+            ret_value = errno;
+            break;
+        }
+    }
 
     p_delete(&addr);
 
-    return EXIT_SUCCESS;
+    return ret_value;
 }
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