fix error reporting of string parser

alib2str/src/registry/StringReaderRegistry.cpp

@@ -16,10 +16,15 @@ std::shared_ptr < abstraction::OperationAbstraction > StringReaderRegistry::getA
 		return entry.first ( ss );
 	};
-	const auto & entries = getEntries ( ) [ group ];
+	const auto & entryIterator = getEntries ( ).find ( group );
+	if ( entryIterator == getEntries ( ).end ( ) )
+		throw exception::CommonException ( "Entry " + group + " not available." );
+
+	const auto & entries = entryIterator->second;
+
 	typename ext::deque < std::pair < std::function < bool ( std::istream & ) >, std::unique_ptr < Entry > > >::const_iterator callback = find_if ( entries.begin ( ), entries.end ( ), lambda );
 	if ( callback == entries.end ( ) )
-		throw exception::CommonException ( "Entry " + group + " not available." );
+		throw exception::CommonException ( "No callback handling input found." );
 	return callback->second->getAbstraction ( );
 }