v.io/x/ref/lib/stats: Use verror instead of fmt.Errorf or errors.New

This change replaces uses of fmt.Errorf and errors.New in lib/stats/...
with verror, and fixes up the dependencies.
The format strings are largely the same:

verror.NoExist is used instead of stats.ErrNotFound, and
stats.ErrNoValue (from services/mgmt/stats) is used instead of stats.ErrNoValue

Tested:
	cd v.io
	v23 go test ./...
	v23 test run vanadium-go-test

Change-Id: I8459ce235b6bbd6c551dbb9cdd45b2116970a8ef
5 files changed