Remove Liquid::FilterNotFoundError since it is never raised.

lib/liquid/errors.rb

@@ -50,7 +50,6 @@ module Liquid
 
   class ArgumentError < Error; end
   class ContextError < Error; end
-  class FilterNotFound < Error; end
   class FileSystemError < Error; end
   class StandardError < Error; end
   class SyntaxError < Error; end

lib/liquid/variable.rb

@@ -93,11 +93,7 @@ module Liquid
           end
         end
         filterargs << keyword_args unless keyword_args.empty?
-        begin
+        output = context.invoke(filter[0], output, *filterargs)
-          output = context.invoke(filter[0], output, *filterargs)
-        rescue FilterNotFound
-          raise FilterNotFound, "Error - filter '#{filter[0]}' in '#{@markup.strip}' could not be found."
-        end
       end
     end
   end