[sled agent] Overhaul error handling to be more informative

[smklein]

Fixes #966

General principles:

• Errors should be "self-contained" as much as possible - if a function returns an error, the error alone should provide enough context to identify "where and why the error happened".
• For externally-defined errors, this often means wrapping the error with a string message, name, or other identifying information
• usage of error(transparent) and thiserror's #[from] has been reduced significantly. These attributes are permitted on "high-level" aggregation services, but only when operating on errors that are sufficiently self-describing.

[jgallagher]

Thanks for this! I think this a fantastic improvement.