computestorage APIs stringify their error instead of wrapping it

[TBBle]

Reference: microsoft/go-winio#187 (comment)

In a unit test, I had code that checked an API call failure to detect missing privilege:

if errno, ok := errors.Cause(err).(syscall.Errno); ok && errno == syscall.ERROR_PRIVILEGE_NOT_HELD {

however, adapting it to use computestorage.FormatWritableLayerVhd, that no longer works, and I must use

if strings.Contains(err.Error(), "A required privilege is not held by the client.") {

This is because of the use of fmt.Errorf('...%s', err). If this is changed to errors.Wrapf(err, '...'), then I'd get the result I wanted.

I guess this was an oversight, since computerstorage/helpers.go does use errors.Wrapf and errors.Wrap.

However, I don't want to just put up a PR without discussion, if this was deliberately done to hide/obscure the underlying failure in these APIs.

[dcantah]

@TBBle Was an oversight, I can send a change for this. Thanks!