Print the stack trace when formatting with %+v

[mmikulicic]

Discovered in:
http://dave.cheney.net/2016/06/12/stack-traces-and-the-errors-package

Code based on:
https://github.com/pkg/errors

With a minor but hackish change to still allow %#v to print the structure fields, in order to not break the principle of least surprise. Let me know if there is a better way to strip a type of an interface or invoke the default %#v behaviour on a type that implements fmt.Formatter.

[mjs]

Thanks for this. LGTM but I'll leave the final verdict to @howbazaar

[howbazaar]

I don't understand why this is necessary?

[mmikulicic]

@howbazaar because otherwise fmt.Fprintf(s, "%#v", e) would yield an infinite recursion.

Any better idea to achieve the same formatting of %#v without re-entering the Format method?

[mmikulicic]

moving to #28 due to organization membership change