Return errors as wrapped errors instead of a custom struct

[Luberry]

This will allow us to use errors.Is to detect sub errors instead of string comparisons.

[maxekman]

Definitely something I want too. Thanks for bringing this up.

A PR is welcome if you have the time, not sure when I'll get to it.

[harry-stebbins-dailypay]

fyi @maxekman #113

[maxekman]

Fixed in #113.