[5970] Make it a kind error message

[y-matsuda]

The error message when making a mistake in setting of Doxygen, isn't kind.
You can know the manner of the setting of Doxygen by a help button by this correction.
I'd like to correct like ChangeVision/astah-cpp-reverse-plugin#10.

https://dev.change-vision.com/projects/jude/ticket/5970 (private URL)