inveniogc: consider journal cache subdirs

* Considers also subdirectories under WebJournal cache directory
  when deciding which files to prune.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  inveniogc: consider journal cache subdirs
WebJournal: structured cache

* Structure the cache with directories.  (closes #1544)

Tested-by: Ludmila Marian <ludmila.marian@gmail.com>
WebJournal: better exception handling when caching

* If an exception occurs when writing cache to disk, catch
  the exception, send it by email to admin, and continue
  serving the page to the user.

Tested-by: Ludmila Marian <ludmila.marian@gmail.com>