repl: Fix error() calls with a non-string message breaking the repl
repl: Fix error() calls with a non-string message breaking the repl

Fixes #898

`error()` calls with non-string messages will now output the following:

```
Welcome to the Luvit repl!
> error(function() end)
error with unexpected error message type (function), no stacktrace available
> error()
error with unexpected error message type (nil), no stacktrace available
```

luvit/luvit,zhaozg/luvit,luvit/luvit,zhaozg/luvit