I18n tasks round 2

Thanks to Vagrant, I've added an API for custom key discovery! This now discovers calls such as `error_key` and `error_namespace` (1 per file). :ship: 
Some other missing and unused keys were discovered, new stats are: 9 missing, 39 unused.

NB: This PR removes `VMGuestError`, as it is not used anywhere (while being the only one declaring error_namespace without a message).