chg: downgrade "failed to read channel" from abortmsg to debugmsg

If the server has an uncaught exception, it will exit without being able to
write the channel information. In this case, the client is likely to complain
about "failed to read channel", which looks inconsistent with original hg.
This patch silences the error message and makes uncaught exception behavior
more like original hg. It will help chg to pass test-fileset.t.

contrib/chg/hgclient.c

@@ -106,8 +106,11 b' static void readchannel(hgclient_t *hgc)'
 	assert(hgc);
 
 	ssize_t rsize = recv(hgc->sockfd, &hgc->ctx.ch, sizeof(hgc->ctx.ch), 0);
-	if (rsize != sizeof(hgc->ctx.ch))
-		abortmsg("failed to read channel");
+	if (rsize != sizeof(hgc->ctx.ch)) {
+		/* server would have exception and traceback would be printed */
+		debugmsg("failed to read channel");
+		exit(255);
+	}
 
 	uint32_t datasize_n;
 	rsize = recv(hgc->sockfd, &datasize_n, sizeof(datasize_n), 0);