Proposal: Error handling with pkg/errors

[vcabbage]

Related to #6.

Dave Cheney's https://github.com/pkg/errors allows adding context to errors without losing the original error. It additionally records stack traces for debugging.

I think that this would help bring consistency to the error handling in ToDD and greatly improve the ability to debug issues.

More info on reasoning for the package:
Blog Post: http://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully
GopherCon Presentation: https://youtu.be/lsBF58Q-DnY?list=PL2ntRZ1ySWBdliXelGAItjzTMxy2WQh0P

[Mierdin]

This is a good idea to look at across the board. I know we need to ensure all packages pass relevant error information up the stack, or handle locally if possible (not always doing this well)

I am adding this issue as a member of the Alpha release milestone.