Bot: clearer subcommand not found error

bot/ctx_call.go

@@ -351,20 +351,14 @@ func (ctx *Context) findCommand(parts []string) ([]string, *MethodContext, *Subc
 			return nil, nil, nil, Break
 		}
 
-		return nil, nil, nil, &ErrUnknownCommand{
-			Parts:  parts,
-			Subcmd: s,
-		}
+		return nil, nil, nil, newErrUnknownCommand(s, parts)
 	}
 
 	if ctx.SilentUnknown.Command {
 		return nil, nil, nil, Break
 	}
 
-	return nil, nil, nil, &ErrUnknownCommand{
-		Parts:  parts,
-		Subcmd: ctx.Subcommand,
-	}
+	return nil, nil, nil, newErrUnknownCommand(ctx.Subcommand, parts)
 }
 
 // searchStringAndSlice searches if str is equal to isString or any of the given

bot/error.go

@@ -2,6 +2,7 @@ package bot
 
 import (
 	"errors"
+	"fmt"
 	"strings"
 )
 
@@ -10,15 +11,28 @@ type ErrUnknownCommand struct {
 	Subcmd *Subcommand
 }
 
-func (err *ErrUnknownCommand) Error() string {
-	if len(err.Parts) > 2 {
-		err.Parts = err.Parts[:2]
+func newErrUnknownCommand(s *Subcommand, parts []string) error {
+	if len(parts) > 2 {
+		parts = parts[:2]
 	}
+
+	return &ErrUnknownCommand{
+		Parts:  parts,
+		Subcmd: s,
+	}
+}
+
+func (err *ErrUnknownCommand) Error() string {
 	return UnknownCommandString(err)
 }
 
 var UnknownCommandString = func(err *ErrUnknownCommand) string {
-	return "unknown command: " + strings.Join(err.Parts, " ")
+	// Subcommand check.
+	if err.Subcmd.StructName == "" || len(err.Parts) < 2 {
+		return "unknown command: " + err.Parts[0]
+	}
+
+	return fmt.Sprintf("unknown %s subcommand: %s", err.Parts[0], err.Parts[1])
 }
 
 var (