6747 Checks in opac-export

6747 Checks in opac-export

Added check on returned marc. If record does not exist, generate 404. (Prevents 500 errors by Googlebot on deleted records.)
Fixed typo, whitespace. Removed if on op=export. Added check on format.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit 223156ea74233122c35d16100724eddbcd69d66a)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>