Bug 17441: [QA Follow-up] Return value of SendAlerts

Bug 17441: [QA Follow-up] Return value of SendAlerts

This patch makes the return value of SendAlerts more consistent.
It returns 1 on success, or undef || { error => 'msg' } on failure.
Needed to adjust one test in Letters.t too.
Adjusted one typo along the way (seleted).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested by running Letters.t.
Also tested SendAlerts from the interface with AutoEmailOpacUser and
memberentry (adding new patron).

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>