create translated underlays in mdwn format•••These are for use by wikis where the primary language is not English.
On such a wiki, it makes sense to use an underlay has the source for pages
in the native language.
pagespec error/failure distinction and error display by inline•••* Add IkiWiki::ErrorReason objects, and modify pagespecs to return
them in cases where they fail to match due to a configuration or syntax
error.
* inline: Display a handy error message if the inline cannot display any
pages due to such an error.
This is perhaps somewhat incomplete, as other users of pagespecs do not
display the error, and will eventually need similar modifications to inline.
I should probably factor out a pagespec_match_all function and make it throw
ErrorReasons.