better failed test message when no annotation channel

tests/datarule.cpp

@@ -133,7 +133,7 @@ void check_sentences(int sentence_i, const Corpus2::Tagset& tagset,
 		BOOST_REQUIRE(annotated);
 		foreach (const Corpus2::AnnotatedSentence::chan_map_t::value_type& v, expected_annotated->all_channels()) {
 			std::string channel_name = v.first;
-			BOOST_REQUIRE(annotated->has_channel(channel_name));
+			BOOST_REQUIRE_MESSAGE(annotated->has_channel(channel_name), "Expected channel " + channel_name);
 			const Corpus2::AnnotationChannel& expected_channel = v.second;
 			std::string expected_channel_data = channel_name + ":" + expected_channel.dump_alpha();
 			Corpus2::AnnotationChannel& channel = annotated->get_channel(channel_name);