v.io/x/ref/services/mgmt:  Use verror instead of fmt.Errorf

This is one of a number of changes to convert code to use verror rather than
fmt.Errorf.  We're frist targeting modules with errors that are likely to pass
over RPC boundaries.

For the moment, I made the error variables local.

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

Change-Id: Ibdc9f90eac516a819fa885c033afe3bd5e2cd757
4 files changed