created_before/after: throw error if specified page does not exist

This assumes that no ctime means the page doesn't exist, which seems
reasonable.

IkiWiki.pm

@@ -2056,7 +2056,7 @@ sub match_created_before ($$;@) {
 		}
 	}
 	else {
-		return IkiWiki::FailReason->new("$testpage has no ctime");
+		return IkiWiki::ErrorReason->new("$testpage does not exist");
 	}
 }
 
@@ -2076,7 +2076,7 @@ sub match_created_after ($$;@) {
 		}
 	}
 	else {
-		return IkiWiki::FailReason->new("$testpage has no ctime");
+		return IkiWiki::ErrorReason->new("$testpage does not exist");
 	}
 }