v.io/x/ref/services/mounttable/lib: Use verror insetad of fmt.Errof or errors.New

This change replaces uses of fmt.Errorf or errors.New with verror in
v.io/x/ref/services/mounttable/lib.

Error messages are kept as similar as possible, typically only adding the
prefix {1:}{2:} (to get the server and RPC name prefixes).

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

Change-Id: I6ee81b288868f94645de4179a7e40e41f2536c7e
2 files changed