gitops-pusher: friendlier error on invalid subcommand

Aid diagnosis of invalid subcommands passed to the gitops-pusher,
particularly in the context of CI where the cause of the error is
non-obvious; an invalid subcommand previously rendered the following
error:

terminal command () doesn't define an Exec function

Fixes #11842.

Signed-off-by: Matthew Huxtable <git@tigermatt.uk>

cmd/gitops-pusher/gitops-pusher.go

@@ -11,6 +11,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"log"
@@ -216,6 +217,14 @@ func main() {
 	}
 
 	if err := root.Parse(os.Args[1:]); err != nil {
+		if noexec := (ffcli.NoExecError{}); errors.As(err, &noexec) {
+			if args := noexec.Command.FlagSet.Args(); len(args) > 0 {
+				log.Fatalf("invalid subcommand %s, see -help for available subcommands", args[0])
+			} else {
+				log.Fatal("missing subcommand, see -help for available subcommands")
+			}
+		}
+
 		log.Fatal(err)
 	}