`/plugins reload non-existent-plugin` printed:
```
Failed to reload plugin: non-existent-plugin, `:^C
```
There were two mistakes:
error_message instead of error_message->str was passed to cons_show().
And in case of failing to unload the plugin due to not finding it in the
hash table it didn't print an error.
This bug was introduced in cc697de05.
2ab9a306ab