ProcedureKitError string conversion memory leak fix (Swift bug) (#661) (#660)
ProcedureKitError string conversion memory leak fix (Swift bug) (#661) (#660)

Swift leaks memory when converting `ProcedureKitError` to a string unless it conforms to `CustomStringConvertible` and provides its own `var description: String { get }` implementation. (Affects at least: Swift 3.0.1 & Xcode 8.1) Symptoms: Malloc 48 byte leaks
This workaround eliminates the leaks.
danthorpe/Operations,ProcedureKit/ProcedureKit,ProcedureKit/ProcedureKit,ProcedureKit/ProcedureKit,danthorpe/Operations,danthorpe/Operations