akamai: replacing error assertions with errors.As (#5127)

errors.As checks for a specific error in a wrapped error chain
(see https://golang.org/pkg/errors/#As) as opposed to asserting
that an error is of a specific type.

Part of #5010

akamai/cache-client.go

@@ -254,7 +254,8 @@ func (cpc *CachePurgeClient) purgeBatch(urls []string) error {
 
 		err := cpc.purge(urls)
 		if err != nil {
-			if _, ok := err.(errFatal); ok {
+			var errorFatal errFatal
+			if errors.As(err, &errorFatal) {
 				cpc.purges.WithLabelValues("fatal failure").Inc()
 				return err
 			}