This commit applies two bulk changes to the grpc error reporting
mechanisms:
(1.) Error strings for errors that originate within grpc are prefixed
with the package name for better clarity for where they originate
since they could percolate up in the users call chains to the
originator.
(2.) Errors that are, in fact, singletons have been converted from
fmt.Errorf to errors.New and assigned as package-level variables.
This bodes particularly well for enabling API customers to elect to
handle these errors upon receipt via equality comparison. This had
been previous impossible with the original API.
Supplementarily, ``gofmt -w -s=true`` has been run on the repository to
cleanup residual defects, and it has detected and repaired a few.
TEST=Manual go test ./...
iamqizhao
Matt T. Proud
David Symonds