ctr: return explicit errors for flags unsupported by transfer service

ctr currently silently ignores several flags by default (without --local) and
the user can't know which flags are supported until they see the code.
This commit fixes ctr to return an explicit error when it finds an unsupported
flag.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

cmd/ctr/commands/images/pull.go

@@ -100,6 +100,15 @@ command. As part of this process, we do the following:
 		defer cancel()
 
 		if !context.Bool("local") {
+			unsupportedFlags := []string{"max-concurrent-downloads", "print-chainid",
+				"skip-verify", "tlscacert", "tlscert", "tlskey", "http-dump", "http-trace", // RegistryFlags
+			}
+			for _, s := range unsupportedFlags {
+				if context.IsSet(s) {
+					return fmt.Errorf("\"--%s\" requires \"--local\" flag", s)
+				}
+			}
+
 			ch, err := commands.NewStaticCredentials(ctx, context, ref)
 			if err != nil {
 				return err