Unknown data readers in non-selected reader conditional paths can prevent reading

As reported in #1101, data readers which do not exist in non-selected reader conditional forms can prevent reading completely:

```
basilisp.user=> #?(:cljs #js [] :default [])


  exception: <class 'basilisp.lang.reader.SyntaxError'>
    message: No data reader found for tag #js
       line: 1:15
```