New method HasMsg() is added in Message_MsgFile class to check if message with given key is registered.
That method is used in Message_Algorithm to check if message is defined on given level of class hierarchy.
Generation of error message in Message_MsgFile::Msg() is revised: now it includes a message key and gets added to the registry, to avoid re-generation in case of multiple requests.
Access to message registry maintained by the Message_MsgFile class is protected from concurrent access by mutex.
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