Retriable errors and backoffs

I'm [working on a tool that uses fetcher.go, and also calls GetProofByHash](https://github.com/aarongable/ctaudit/pull/1). I'd like to configure it to do backoffs and retries. I see that fetcher.go uses a `Backoff` struct provided internally:

https://github.com/google/certificate-transparency-go/blob/6ecca400d5e42ae3f26ad5c27cea9f907474024d/scanner/fetcher.go#L273-L292

But I think that usage might be broken. `Backoff` will only retry on grpc error codes that are retriable, plus the `RetriableError` struct. The GetRawEntries call I linked to is implemented by `jsonclient.GetAndParse`, which [returns RspErrors](https://github.com/google/certificate-transparency-go/blob/6ecca400d5e42ae3f26ad5c27cea9f907474024d/jsonclient/client.go#L169-L209). Those RspErrors don't match any of the conditions for retries.

I'd like to propose this: In addition to / instead of backoff.RetriableError, the backoff package should check for some interface, e.g. `Retryable() bool`. Then `jsonclient.RspError` can implement `Retryable()` as: a RspError is retryable if its wrapped error is a net.Timeout error OR the HTTP status code is >500.

What do you think?

Start by reading scanner/fetcher.go around the Backoff usage and jsonclient/client.go around GetAndParse and RspError. Trace how retry decisions are made for gRPC errors, RetriableError, and RspErrors; done means the proposed retry behavior for timeout and HTTP status errors is defined and covered consistently across these entry points.

Written by the indexing model from the issue text.