This will allow implementors to use these errors to detect
if an error has occurred, rather than using string comparison.

This also adds an ErrorDetails field to the Root object, which
is mostly for debugging.

Add some comments and update changelog

Exposes a soup.Error that can be inspected to see what kind of error
you are dealing with, then be able to further inspect the Error()
method to see if there is any more information for you.

Avoids breaking backwards compatibility by not changing the type of
Error in the Root object, so inspecting the returned err should
still work in other projects.