veyron2/verror2: Add a "Cancelled" error.
veyron2/verror2: Add a "Cancelled" error.

Add a "Cancelled" error to the list of standard error codes.
It seems likely this will come up in various RPC interfaces.

Change-Id: I25cd6f3540b78b7dbbfbe0fbebf2c0196999716e

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23