linux, linux/shim: remove error definitions

Since we now have a common set of error definitions, mapped to existing
error codes, we no longer need the specialized error codes used for
interaction with linux processes. The main issue was that string
matching was being used to map these to useful error codes. With this
change, we use errors defined in the `errdefs` package, which map
cleanly to GRPC error codes and are recoverable on either side of the
request.

The main focus of this PR was in removin these from the shim. We may
need follow ups to ensure error codes are preserved by the `Tasks`
service.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

linux/shim/exec.go

@@ -164,7 +164,7 @@ func (e *execProcess) Resize(ws console.WinSize) error {
 
 func (e *execProcess) Kill(ctx context.Context, sig uint32, _ bool) error {
 	if err := unix.Kill(e.pid, syscall.Signal(sig)); err != nil {
-		return checkKillError(err)
+		return errors.Wrapf(checkKillError(err), "exec kill error")
 	}
 	return nil
 }