Add scenario to show issue #35
Add some context to returned errors

This makes it much easier to track down the cause of errors.  We are
doing this primarily to help investigate issue #35.
Add special case to handle the error interface

The error interface is an internal interface - so we are not going to
learn about it by reading any files...

This helps with issue #35, but reveals another problem.
Start limiting stdlib to files for the platform

Instead of taking across and processing all files, we are now just
dealing with the files for the current GOOS/GOARCH combination.  This
work isn't complete as we only process file names - not the embedded
build constraints.  This is enough to make it possible to mock the net
package though.

Fixes #35
Finally fix lib test

Been meaning to do this for a while ... we just need to skip the
badpkg from the Go testdata - as it isn't supposed to be able to be
parsed successfully.