Keep memory errors as memory errors

Allow memory errors to be raised through the API (throwing the
error with the memory error message); error in external allocations
raises a memory error; memory errors in coroutines are re-raised
as memory errors.

lapi.c

@@ -1195,9 +1195,15 @@ LUA_API int lua_gc (lua_State *L, int what, ...) {
 
 
 LUA_API int lua_error (lua_State *L) {
+  TValue *errobj;
   lua_lock(L);
+  errobj = s2v(L->top - 1);
   api_checknelems(L, 1);
-  luaG_errormsg(L);
+  /* error object is the memory error message? */
+  if (ttisshrstring(errobj) && eqshrstr(tsvalue(errobj), G(L)->memerrmsg))
+    luaM_error(L);  /* raise a memory error */
+  else
+    luaG_errormsg(L);  /* raise a regular error */
   /* code unreachable; will unlock when control actually leaves the kernel */
   return 0;  /* to avoid warnings */
 }