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.

1 files changed, 6 insertions, 3 deletions

diff --git a/doc/plugins/write.mdwn b/doc/plugins/write.mdwn
index 11ed312ae..23df01ca7 100644
--- a/doc/plugins/write.mdwn
+++ b/doc/plugins/write.mdwn
@@ -890,9 +890,12 @@ It's also possible to write plugins that add new functions to
 IkiWiki::PageSpec package, that is named `match_foo`, where "foo()" is
 how it will be accessed in a [[ikiwiki/PageSpec]]. The function will be passed
 two parameters: The name of the page being matched, and the thing to match
-against. It may also be passed additional, named parameters. It should return
-a IkiWiki::SuccessReason object if the match succeeds, or an
-IkiWiki::FailReason object if the match fails.
+against. It may also be passed additional, named parameters.
+
+It should return a IkiWiki::SuccessReason object if the match succeeds, or
+an IkiWiki::FailReason object if the match fails. If the match cannot be
+attempted at all, for any page, it can instead return an
+IkiWiki::ErrorReason object explaining why.
 
 ### Setup plugins