Saving still 'works' when not logged in

[samwilson]

When not logged in, clicking the save-coordinates button advances the progress bar, disables the button, but doesn't actually save anything. A post request is made, which results in an error visible in the browser console, but no user-facing warning is shown.

Possibly, the idea in #68 would also fix for this bug, in that it wouldn't be possible to click the save button without being logged in.

(Thanks for a brilliant tool by the way!)