Add http_error callback

Allows user code to display its own error status responses or just log
the errors. Is called even in cases where begin_request is not called.

mongoose.c

@@ -915,6 +915,8 @@ static void send_http_error(struct mg_connection *conn, int status,
   int len = 0;
 
   conn->status_code = status;
+  if (conn->ctx->callbacks.http_error == NULL ||
+      conn->ctx->callbacks.http_error(conn, status)) {
     buf[0] = '\0';
 
     // Errors 1xx, 204 and 304 MUST NOT send a body
@@ -933,6 +935,7 @@ static void send_http_error(struct mg_connection *conn, int status,
               "Connection: %s\r\n\r\n", status, reason, len,
               suggest_connection_header(conn));
     conn->num_bytes_sent += mg_printf(conn, "%s", buf);
+  }
 }
 
 #if defined(_WIN32) && !defined(__SYMBIAN32__)

mongoose.h

@@ -67,6 +67,7 @@ struct mg_callbacks {
                              const char *path, size_t *data_len);
   void (*init_lua)(struct mg_connection *, void *lua_context);
   void (*upload)(struct mg_connection *, const char *file_name);
+  int  (*http_error)(struct mg_connection *, int status);
 };
 
 // Start web server.