Display error for unknown tags in home.lang

Add all available tags to en-GB/mozorg/home.lang, display unknown tags
only in errors view.

Rationale:
* I realized 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
for now.
Merge pull request #64 from flodolo/error_tags

Error views: display unknown tags in home.lang