Fix Error token in common Objective-C module (#1406)

Certain objects that were extracted from the Objective-C lexer in #1378
needed to be prefaced with a `base` variable. This included the token
classes. This was not done for `Error`. This commit fixes that mistake.

lib/rouge/lexers/objective_c/common.rb

@@ -44,7 +44,7 @@ def self.extended(base)
             elsif base.at_builtins.include? m[1]
               token base::Name::Builtin
             else
-              token Error
+              token base::Error
             end
           end
 

spec/visual/samples/objective_c

@@ -173,3 +173,6 @@ ViewController *controller = [[ViewController alloc] initWithDocument:document c
     builder.thumbnailBarMode = ThumbnailBarModeScrollable;
     builder.pageLabelEnabled = NO;
 }]];
+
+// Invalid Objective-C keyword
+@FOO