Getting an API exception when opening the glossary and an update is available

Eg when accessing `index.php?module=API&action=glossary&idSite=1&period=day&date=yesterday` and a DB update is available, I get an API error like this:

```xml
<result>
<error message="Database Upgrade Required. Your Piwik database is out-of-date, and must be upgraded before you can continue."/>
</result>
```

It should instead show the upgrade screen.

