Fix /plugins reload error message

`/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.

src/command/cmd_funcs.c

@@ -7197,7 +7197,7 @@ cmd_plugins_reload(ProfWin* window, const char* const command, gchar** args)
     if (res) {
         cons_show("Reloaded plugin: %s", args[1]);
     } else {
-        cons_show("Failed to reload plugin: %s, %s", args[1], error_message);
+        cons_show("Failed to reload plugin: %s, %s.", args[1], error_message->str);
     }
     g_string_free(error_message, TRUE);
 

src/plugins/plugins.c

@@ -354,6 +354,9 @@ plugins_reload(const char* const name, GString* error_message)
     gboolean res = plugins_unload(name);
     if (res) {
         res = plugins_load(name, error_message);
+    } else {
+        log_info("Failed to reload plugin: %s, not loaded", name);
+        g_string_assign(error_message, "cannot unload");
     }
 
     return res;