Fix uncaught error in ServerException

[marfavi]

Use Failures instead of exceptions where possible

Make message in ServerFailure.fromResponse nullable; catch FormatException (happens when empty string gets parsed)

Remove interface fo OpeningHoursRemoteDataSource, since its implementation was in the same file

Improve logic that generates opening hours from list of shifts; now all days that don't have shifts are closed (no assumptions are made about what days are closed)

[fremartini]

@marfavi @TTA777 https://github.com/AnalogIO/coffeecard_app/pull/404

[fremartini]

closed by #404