WebSearch: proper re-raise in RSS handling

* Re-raises properly if an exception occurs during serving
  of the cache in a RSS request. (closes #2084)

* Adds safer handling of files in case of exceptions.

Reported-by: Thorsten Schwander <thorsten.schwander@gmail.com>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>