better error message when a colon cannot be found in the drn file

src/storm-parsers/parser/DirectEncodingParser.cpp

@@ -349,7 +349,7 @@ namespace storm {
                 } else {
                     // New transition
                     size_t posColon = line.find(':');
-                    STORM_LOG_THROW(posColon != std::string::npos, storm::exceptions::WrongFormatException, "':' not found.");
+                    STORM_LOG_THROW(posColon != std::string::npos, storm::exceptions::WrongFormatException, "':' not found in '" << line << "'.");
                     size_t target = parseNumber<size_t>(line.substr(2, posColon - 3));
                     std::string valueStr = line.substr(posColon + 2);
                     ValueType value = parseValue(valueStr, placeholders, valueParser);