Bug 10630: give DelBookseller() a return value

Bug 10630: give DelBookseller() a return value

This patch makes DelBookseller returns the number of suppliers it has
deleted or undef if an error has occurred.

It also fixes a test which doesn't pass before in t/db_dependent/Bookseller.t

To test:
prove t/db_dependent/Bookseller.t
t/db_dependent/Bookseller.t .. 1/54
All tests successful.
Files=1, Tests=54,  1 wallclock secs ( 0.02 usr  0.00 sys +  0.48 cusr  0.02 csys =  0.52 CPU)
Result: PASS

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Logical change and makes another test pass :)

Signed-off-by: Galen Charlton <gmc@esilibrary.com>