Gracefully handle errors and timeouts in backend requests

[rod-glover]

This means two things:

• catch them
• provide a graceful user experience when they occur

Further notes:

• Currently, the app shows an eternal spinner when a date is chosen for which there is no data. This is easy to do; just pick a date after the current date. Things should be done to improve this:
  • Disallow date choices after present date ... but note baseline month is valid regardless of current date.
  • Apply timeouts to requests, and provide nice messages to user.
  • Distinguish between "no data" and timeout. This is not currently done.

[rod-glover]

Do this after #102