Improve error view: link locale, show missing strings

### Link to locale's langchecker
I use target, which is not ideal, but this is also a view for internal use.

### Missing strings
Webdashboard show them in a weird way (only a 0 is displayed), so most localizers don't realize what it means. In most cases, that's an error. This is an [example](http://viewvc.svn.mozilla.org/vc?revision=128637&view=revision) of what I fixed this morning thanks to this view.