pagespec error/failure distinction and error display by inline

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.