More human-readable errors

[helen]

I'd like to write and implement more human-readable error messages instead of just reporting back the provided error code and messages. So for instance, instead of showing 429: Too many requests in the banner, we could try:

You have reached the limit for classifications on your account. (Error code 429)

And so on and so forth, tailoring them for each combination of error code and service and particular UI and potentially providing help if it makes sense in any situation.

Previously: #3

[jeffpaul]

As we're looking to release v1.5.0 in the next week or so and with no linked PR for this issue, I'm going to move this to Future Release until we're able to work up a PR for it.