Better error message for INSTANCE: if second arg is not a mixin

basis/debugger/debugger.factor

@@ -3,13 +3,13 @@
 USING: slots arrays definitions generic hashtables summary io
 kernel math namespaces make prettyprint prettyprint.config
 sequences assocs sequences.private strings io.styles io.files
-vectors words system splitting math.parser classes.tuple
-continuations continuations.private combinators generic.math
-classes.builtin classes compiler.units generic.standard vocabs
-init kernel.private io.encodings accessors math.order
-destructors source-files parser classes.tuple.parser
-effects.parser lexer compiler.errors generic.parser
-strings.parser ;
+vectors words system splitting math.parser classes.mixin
+classes.tuple continuations continuations.private combinators
+generic.math classes.builtin classes compiler.units
+generic.standard vocabs init kernel.private io.encodings
+accessors math.order destructors source-files parser
+classes.tuple.parser effects.parser lexer compiler.errors
+generic.parser strings.parser ;
 IN: debugger
 
 GENERIC: error. ( error -- )
@@ -327,3 +327,5 @@ M: bad-effect summary
 M: bad-escape summary drop "Bad escape code" ;
 
 M: bad-literal-tuple summary drop "Bad literal tuple" ;
+
+M: check-mixin-class summary drop "Not a mixin class" ;

core/classes/mixin/mixin.factor

@@ -25,7 +25,7 @@ M: mixin-class rank-class drop 3 ;
         bi
     ] if ;
 
-TUPLE: check-mixin-class mixin ;
+TUPLE: check-mixin-class class ;
 
 : check-mixin-class ( mixin -- mixin )
     dup mixin-class? [