classes.mixin: better name for the type checking error

basis/debugger/debugger.factor

@@ -370,7 +370,7 @@ M: bad-escape error.
 
 M: bad-literal-tuple summary drop "Bad literal tuple" ;
 
-M: check-mixin-class-error summary drop "Not a mixin class" ;
+M: not-a-mixin-class summary drop "Not a mixin class" ;
 
 M: not-found-in-roots summary
     path>> "Cannot resolve vocab: " prepend ;

core/classes/mixin/mixin.factor

@@ -18,12 +18,10 @@ M: mixin-class reset-class
 
 M: mixin-class rank-class drop 8 ;
 
-ERROR: check-mixin-class-error class ;
+ERROR: not-a-mixin-class class ;
 
 : check-mixin-class ( mixin -- mixin )
-    dup mixin-class? [
-        check-mixin-class-error
-    ] unless ;
+    dup mixin-class? [ not-a-mixin-class ] unless ;
 
 <PRIVATE