Bug 12858: Add error handling to Syndetics Index

Bug 12858: Add error handling to Syndetics Index

* Syndetics routines include a statement to check that the returned
  content from Syndetics is xml. The get_syndetics_index routine was
  missing this check and so when a 'not found' html page was returned
  the opac-detail page would take a long time to load whilst xml::simple
  attempted to parse the large html document.

  Test Plan
  1. Enable Syndetics indexes on opac.
  2. Head over to an opac-detail page on the opac.
  3. Remove the ISBN from the same item in the staff client.
  4. Reload the opac-detail page for the item noticeing a much slower
  page load.
  5. Apply the patch
  6. Reload the opac-detail page for the item and note that it now laods
  in a reasonable timescale again.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>
(cherry picked from commit 8ac1c243fe86fc9a77f54f1d21d3448491d27da0)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>