Error handling in frontend

[howamith]

At the time of writing the frontend application doesn't have any sort of error handling (or even a means to signify to the user that something went wrong 😅) - add this in. Vuetify's Banners and Snackbars will be handy here.

Think about what the frontend app will do if the API returns an error or if it's down entirely.

Note: depends on cron-mon-io/cron-mon#18