Consolidate Debuggable.stackTrace with the rest of the error types

[zdnk]

References: https://github.com/vapor/vapor/issues/1804

The stackTrace was optional and was unreachable by trying cast error: Error to Debuggable, it was always returning nil.

[MrLotU]

Ah. Of course. Something to keep in mind for Vapor 4 in that case.