Improvement Error Handling

[Satoshi Hachiya]

I think it is not necessary to implement the following:

} catch let sharingFlowError as SharingFlowError {
    completion?(sharingFlowResult: .Failure(self.imagePath, sharingFlowError))
    return

https://github.com/JPMartha/EggsBenedict/blob/develop/EggsBenedict/SharingFlow.swift#L114-L116

The SharingFlowError enumeration adopts the ErrorType protocol.