Adds default parameters to the constructor of DflException
https://github.com/qchikara/dfl/commit/a48b720d30b0eaf3b8203775a78781033e25cc5c
(It is convenient that an exception has the location information  where
it was created and thrown. This patch adds default parameters, __FILE__
and __LINE__, to DflException's constructor like other Exception
classes.)