Bug 11329: Check for MARC record existence in opac-showmarc

Bug 11329: Check for MARC record existence in opac-showmarc

Instead of just running as_formatted, check if GetMarcBiblio returned
a reference. If you e.g. did not pass an id, return 404 instead of 500.
Consistent with opac-export.pl

Test plan:
[1] Run opac-showmarc.pl with valid biblionumber in id parameter.
[2] Remove id parameter from URL. You should get a 404 now.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>