Bug 1034780 - Show message when update found after forced checking

In order to avoid confusion, if gecko or apps update checker returns "check-complete", hide the "Checking for update..." message and show "Update found". 
This line exceeds 80 characters. You could format it as:
```
var l10nValues = [
  'check-complete',
  'retry-when-online',
  'already-latest-version',
  'no-updates'
];
```