Better error messages

[rasagy]

@ajithranka Let’s use this ticket to figure out if we want to change the copy in some of the error messages, and then sync up with @Rub21 to help us get them working on the server side. Feel free to add more:

Error	Reason	New message	Status
JOSM not connected	Because JOSM isn’t open?
Invalid access token	Need to login again since token expires after 24 hrs
Completed/blocked task	Someone has previewed all remaining features
Completed tasks	Nothing to Preview

We should also add a wiki in the To-fix repo that explains common errors & fixes in detail (and link to it instead of the /issues link, which can sit in the wiki). :)

cc @geohacker @planemad @batpad

[ajithranka]

Error messages need to change in to-fix-backend. See src/configs/config.js.