feat: better convert usage

internal/cli/marshaler.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"fmt"
+	"reflect"
 
 	"git.maximhutz.com/max/lambda/pkg/codec"
 )
@@ -29,7 +30,9 @@ func (c convertedMarshaler[T]) Decode(s string) (Repr, error) {
 func (c convertedMarshaler[T]) Encode(r Repr) (string, error) {
 	t, ok := r.Data().(T)
 	if !ok {
-		return "", fmt.Errorf("could not parse '%v' as 'string'", t)
+		dataType := reflect.TypeOf(r.Data())
+		allowedType := reflect.TypeFor[T]()
+		return "", fmt.Errorf("marshaler for '%s' cannot parse '%s'", allowedType, dataType)
 	}
 
 	return c.codec.Encode(t)