Address feedback for error handling

[C-Loftus]

• type the t function so there is intellisense and build errors if it is called with a message that doesnt exist in the locales
• test the t function and the locale json files to make sure all locales have the same key names
• Change the message when there are no discovery datasets
• Check caching issues with nginx
  • (I think this is coming from a difference in the backend now, not the frontend)
• Add machine translation strings for each of the locales for the new error message

Unfinished. Will merge into #95 once finished (and then we can merge that into main), but separated for easier review