vocabs: On use-vocab we should throw an error if the vocabulary does not exist.

Also ui.pixel-formats.private does not exist so remove that.

Fixes #2298.

basis/ui/backend/gtk/gtk.factor

@@ -9,7 +9,7 @@ math.vectors namespaces sequences strings system threads ui ui.backend
 ui.backend.gtk.input-methods ui.backend.gtk.io ui.backend.x11.keys
 ui.clipboards ui.event-loop ui.gadgets ui.gadgets.private
 ui.gadgets.worlds ui.gestures ui.pixel-formats
-ui.pixel-formats.private ui.private vocabs.loader ;
+ui.private vocabs.loader ;
 IN: ui.backend.gtk
 
 SINGLETON: gtk-ui-backend

core/vocabs/parser/parser.factor

@@ -115,7 +115,7 @@ ERROR: unbalanced-private-declaration vocab ;
         vocab-name "Already using ``" "'' vocabulary" surround note.
     ] [
         manifest get
-        [ [ load-vocab ] dip search-vocabs>> push ]
+        [ [ ?load-vocab ] dip search-vocabs>> push ]
         [ [ vocab-name ] dip search-vocab-names>> adjoin ]
         2bi
     ] if ;

core/vocabs/vocabs.factor

@@ -47,6 +47,11 @@ M: vocab lookup-vocab ;
 
 M: object lookup-vocab vocab-name dictionary get at ;
 
+ERROR: no-vocab-named name ;
+
+: ?lookup-vocab ( vocab-spec -- vocab )
+    dup lookup-vocab [ nip ] [ no-vocab-named ] if* ;
+
 GENERIC: vocab-words-assoc ( vocab-spec -- assoc/f )
 
 M: vocab vocab-words-assoc words>> ;
@@ -156,3 +161,6 @@ M: string require
 
 : load-vocab ( name -- vocab )
     [ require ] [ lookup-vocab ] bi ;
+
+: ?load-vocab ( name -- vocab )
+    [ require ] [ ?lookup-vocab ] bi ;