Proposal using pkg/errors to allow stack trace on unexpected errors

[maleck13]

Using the default error_handler middleware, I wanted to allow for printing a stack trace to the log when an unexpected error happens as it is these kind of errors that you want to know where the error occurred. I made this change locally by using github.com/pkg/errors to change NewErrorClass to use
https://godoc.org/github.com/pkg/errors#Wrap and then changed the error_handler middleware to use https://godoc.org/github.com/pkg/errors#Cause. This allowed a very simple change to the log statement to print the stack trace.
I wondered if this would be something you might like to include on master. If so I will create a PR with my changes

[raphael]

That seems like a good idea to me, feel free to submit a PR and thank you!

[raphael]

Thanks for the PR!