Error views: display unknown tags in home.lang

Add all available tags to en-GB/mozorg/home.lang, display unknown tags in errors view.

Rationale:
* I realized today that “da” never displayed the addons tile because of a misspell (addon2013 instead of addons2013). 
* Limiting this to errors view because localizers don’t manage tags.
* No point in spending time checking all other files, just home.lang