Bug 4189 : Searching z39.50 without selecting any servers results in error message

This patch makes z3950_search.pl handle the situation properly, and z3950_search.tmpl
show a proper message instead of a program error. All this, in case the javascript
checks that alert the user (that at least one target has to be chosen)  are by-passed.

Regards
To+

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Fixed small translation problem in javascript alert message.
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>