Ability to specify multiple arguments to \cfg{html-template-fragment};

Ability to specify multiple arguments to \cfg{html-template-fragment};
Halibut will output fragment names in all specified formats. (I forget now
precisely why I thought this was necessary, but it seems potentially useful.)

Also ensure that legal fragment names are generated even if none of the
characters from the original turn out to be legal (e.g., %k with an entirely
numeric keyword), and correct an untruth I inserted in the documentation of
this.

(This commit hits more than just the HTML backend as I've generalised an error
message, and fixed a fault in the info backend's error handling while there.)

git-svn-id: svn://svn.tartarus.org/sgt/halibut@5457 cda61777-01e9-0310-a592-d414129be87e