API: Distinguish between internal/external resources, especially in the context of errors

[smklein]

Context: #457 (comment)

The external::Error type references a ResourceType enum. This enum is a necessary part of certain errors, such as "Not found".

Although expressive and useful for externally-facing APIs, this unfortunately has resulted in a conflated resource type for internally-facing APIs.

We should figure out a way to use these expressive error types while separating the notion of an "internally visible" and "externally visible" resource.

(As a concrete example: "Disks" and "Instances" are externally-facing resources. "SagaDbg", "DownloadArtifact", "MetricProducer" are all internally-facing)