Handle NewDay Unavailability

[BenjaminEHowe]

NewDay's online portal seems to be somewhat unreliable. See the below stack trace for what happens when it goes "down":

io.yamm.backend.YAMMRuntimeException: System currently unavailable, please try again.
    at io.yamm.backend.providers.NewDay.authenticate(NewDay.java:172)
    at io.yamm.backend.providers.NewDay.callEndpoint(NewDay.java:232)
    at io.yamm.backend.providers.NewDay.callEndpoint(NewDay.java:182)
    at io.yamm.backend.providers.NewDay.callAccountSummaryEndpoint(NewDay.java:249)
    at io.yamm.backend.providers.NewDay.getAvailableToSpend(NewDay.java:415)
    at io.yamm.client.DataHandler.accountToJSON(DataHandler.java:146)
    at io.yamm.client.DataHandler.accountsToJSON(DataHandler.java:169)
    at io.yamm.client.Webserver.serve(Webserver.java:240)
    at fi.iki.elonen.NanoHTTPD$HTTPSession.execute(NanoHTTPD.java:945)
    at fi.iki.elonen.NanoHTTPD$ClientHandler.run(NanoHTTPD.java:192)
    at java.lang.Thread.run(Thread.java:748)

It would be good to mitigate this somehow, but I'm not sure how! One option would be to just return cached data (and show an error to the user) but this would fail if the caches are empty. The YAMM web app probably needs a "data not available" state, or similar.

[BenjaminEHowe]

Examples of previous unavailability:

• December 2017
• April 2018 (see also stack trace above)

[BenjaminEHowe]

They had a second outage in April 2018.