Logs a warning message. Such a message indicates a problem
to the user. The problem doesn't affect the course of
DTDDoc but is worth noting because it's the result of a
an action or the data of the user.
Logs an error message. Such a message indicates something
wrong happened inside DTDDoc. This thing may affect the
course of execution of DTDDoc. That is, if it happens, the
result of the execution is not guaranteed.