storage: do not share *roachpb.Error
storage: do not share *roachpb.Error

Previous code was returning the same `*roachpb.Error` to multiple
clients blocked on a LeaderLease request, which cause data races.

In the absence of similar issues present elsewhere, this fixes #6100
and fixes #6099.

maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,petermattis/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,paperstreet/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,maxlang/cockroach,sunya123/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,arjun4084346/cockroach,petermattis/cockroach,veteranlu/cockroach,maxlang/cockroach,glycerine/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,glycerine/cockroach,paperstreet/cockroach,sunya123/cockroach,glycerine/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,petermattis/cockroach,petermattis/cockroach,sunya123/cockroach,glycerine/cockroach,paperstreet/cockroach,cockroachdb/cockroach,sunya123/cockroach,maxlang/cockroach,petermattis/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,glycerine/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,cockroachdb/cockroach,veteranlu/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,paperstreet/cockroach,maxlang/cockroach,vaibhavarora/cockroach,sunya123/cockroach,arjun4084346/cockroach,paperstreet/cockroach,cockroachdb/cockroach,veteranlu/cockroach,glycerine/cockroach,arjun4084346/cockroach,paperstreet/cockroach,sunya123/cockroach,petermattis/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,maxlang/cockroach,cockroachdb/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,sunya123/cockroach,veteranlu/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,petermattis/cockroach