Fix `httpClientHandler` to properly distinguish error codes

[dlongley]

See: https://github.com/digitalbazaar/bedrock-jsonld-document-loader/blob/v5.2.0/lib/index.js#L31

This error should be changed to only return a NotFoundError (keeping the message the same for backwards compatibility) on a 404 status code. Otherwise, a different error could be returned. The error should also become a BedrockError that includes the httpStatusCode in the details so it can be processed by callers.

[dlongley]

Addressed by #20.