Prevent hl_thread_id throw error even when we can't get tid (#601)

src/std/thread.c

@@ -812,7 +812,6 @@ HL_PRIM int hl_thread_id() {
 #elif defined(SYS_gettid) && !defined(HL_TVOS)
 	return syscall(SYS_gettid);
 #else
-	hl_error("hl_thread_id() not available for this platform");
 	return -1;
 #endif
 }