sled-agent: Clean up error types and HttpError conversions

I have wondered for a while here if we might want to instead do something where each of the individual error variants has its own Into<HttpError> implementation, and this higher-level thing looks like a match that just calls instance::Error::into(), instance_manager::Error::into(), BundleError::into(), and so forth, so that the conversions (and the corresponding big lists of every variant of every enum) are written in the files where those errors are defined, instead of all here, but I'm not sure OTTOMH whether that would actually end up making sense or not --- there might be some reason that didn't work that I've forgotten about?

This sounds reasonable to me, but honestly I'd prefer we break up these giant error types more significantly - it seems a little hard to believe that we really need to represent both "the sled doesn't have a boot disk" and "failed to resolve a name in internal DNS" in a single error type, to pick an a couple variants of this error. That would presumably force us to move the conversions down at least one layer too.

Originally posted by @jgallagher in https://github.com/oxidecomputer/omicron/issues/11142#issuecomment-5401209495

The issue names no files, tests, or entry points. Start by locating sled-agent's aggregate error types and HttpError conversions, then evaluate whether the errors should be split and their conversions moved to the defining modules. Done means the error structure and conversion ownership have been decided and consistently updated.