improved error handling

[danieleades]

the error handling can be improved.

taking inspiration from

• this blog post on error handling in 'Sled'
• error handling in Alexandrie (@Hirevo)

The idea is that you really have two classes of errors

• local errors - expected in normal operation. usually handled 'locally'
• global errors- some serious issue that should not occur during normal operation. These should generally be propagated all the way up

The idea is to 'double-wrap' the Results such that the inner Errors encode 'local' errors (such as validation errors), while the outer Errors encode 'showstoppers' (such as IO errors).

The downside is the error types get a little ugly (Result<Result<T, LocalError>, GlobalError>).

the upside is that in calling code, a consumer of the error can use '?' to propagate and unwrap the global error upwards, and explicitly handle the so-called local error with a match statement.