Fix error description forwarding

[czechboy0]

Motivation

Fixes apple/swift-openapi-generator#730.

We were not correctly keeping CustomStringConvertible and LocalizedError conformances separate for wrapper errors like ClientError and ServerError. This lead to some user-thrown errors (in handlers, transports, and middlewares) to print less information than the error was actually providing (using a different method).

Modifications

Properly untangle the two printing codepaths, and only call localizedDescription from the wrapper error's errorDescription.

Also made the localizedDescription strings a bit more user-friendly and less detailed, as in some apps these errors might get directly rendered by a UI component that calls localizedDescription.

Result

Error logging should now match adopter expectations.

Test Plan

Added unit tests for {Client,Server}Error printing methods.

[czechboy0]

cc @weissi

[simonjbeaumont]

Looks good, thanks @czechboy0!

Took me a minute to confirm that prettyDescription didn't get in the way again, but I believe this resolves the ask and we can avoid making use of localized description in pretty description going forward too.