Let's make fetch success/error/etc not depend on HTTP status codes?

[ermo]

From an abstract point of view, we're interested in whether a download succeeded or not and what the reason was if it didn't succeed.

The current design assumes that the transport is HTTP and is therefore centered on HTTP status codes, which doesn't map well to @GZGavinZhao moss-fetcher + boulder git support PR.

Ikey suggested that we need to couple this with the error handling in moss.core, so capturing it here for reference and labelling it appropriately.