When no agenda files are defined add current file to agenda files

[tpaixao]

vimOrganizer does this and it makes sense...

Moreover, if a new user is trying out the features, and tries to invoke the TODO dashboard, he will only see error messages.

[Shurakai]

org-mode itself doesn't do it; I assume that they presume users to have their well-defined set of files that they use and want to open from any location.

In emacs org-mode, one can however restrict the agenda to the current buffer by pressing "<" several times. It doesn't matter in this case if the file is in the list or not, see http://orgmode.org/manual/Agenda-files.html#Agenda-files

[Ron89]

I also think the current agenda setting is a bit confusing. As in, I didn't use emacs agenda at all, and before coming across this thread, I don't even know what the error message meant. I think it's better to at least add some information in the documentation and some error handler to make the error Message more informative.