Add notes on timeseries data to Keras example

[drasmuss]

Also added a bit more information to the Converter verification failure error message, and fixed a bug in the logging messages I discovered while looking at this.

[hunse]

I found that test_logging didn't actually format the log messages, i.e. if the format specifiers in a message didn't match the arguments, it wouldn't catch it. So I added the loop at the end to format all the log messages, and make sure they're non-empty. If this looks good to you @drasmuss, I'll merge when the tests pass.

[drasmuss]

Looks good