[MINOR] store HTTP error messages into a chunk array

HTTP error messages were all specific cases handled by an IF.
Now they are all in an array so that it will be easier to add
new ones. Also, the return functions now use chunks as inputs
so that it should be easier to provide alternative return
messages if needed.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 61d58c2..5ee3a53 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -40,10 +40,10 @@
 int process_cli(struct session *t);
 int process_srv(struct session *t);
 
-void client_retnclose(struct session *s, int len, const char *msg);
-void client_return(struct session *s, int len, const char *msg);
+void client_retnclose(struct session *s, const struct chunk *msg);
+void client_return(struct session *s, const struct chunk *msg);
 void srv_close_with_err(struct session *t, int err, int finst,
-			int status, int msglen, const char *msg);
+			int status, const struct chunk *msg);
 
 int produce_content(struct session *s);
 void debug_hdr(const char *dir, struct session *t, const char *start, const char *end);