…error handling

This is not completely automatable here due to some semantic differences between
errors.Wrap and fmt.Errorf with the %w verb; the latter will convert a nil error
into a non-nil error, so this must be guarded with a nil error check.

The mechanical transformation was done by running the following commands:
	gofmt -w -r 'errors.Errorf -> fmt.Errorf' .
	gofmt -w -r 'errors.Wrap(e, m) -> fmt.Errorf(m+": %w", e)' .
	gofmt -w -r 'errors.Wrapf(e, m) -> fmt.Errorf(m+": %w", e)' .
	gofmt -w -r 'errors.Wrapf(e, m, a) -> fmt.Errorf(m+": %w", a, e)' .
	gofmt -w -r 'errors.Wrapf(e, m, a, b) -> fmt.Errorf(m+": %w", a, b, e)' .
	gofmt -w -r 'errors.Wrapf(e, m, a, b, c) -> fmt.Errorf(m+": %w", a, b, c, e)' .
	find . -name '*.go' -exec gsed -i -e 's/"+": %w",/: %w",/g' '{}' \;
	find . -name '*.go' -exec gsed -i -e 's!"github.com/pkg/errors"$!"errors"!g' '{}' \;
	goimports -w .
	go mod tidy
	gofumpt -w .

Replaying that will give the changes here modulo some manual changes that are
made in the next commit. There should be no uses of %w in this change that are
not guarded by a nil-check.

…r handling

This is the manual part of the change and includes some complete removal of errors
and some silently passed bugs resulting from the use of Wrap.

In one place the exact semantics of errors.Cause is implemented to fit in with a
testing strategy.