Bug 17165: Improve heading on vendor search when searching for all vendors in Serials

Bug 17165: Improve heading on vendor search when searching for all vendors in Serials

To test:
1) Go to Serials
2) Click New subscription or edit an existing one
3) Click Search for a vendor
4) Make a search that will return no results (i.e. has a typo etc.)
5) Confirm there is an appropriate message
6) Go back and make a search that will return results (i.e. putting in one letter
    'a' etc.)
7) Confirm that heading is worded better and search term is displayed
8) Go back and make search without entering any search terms
9) Confirm that heading is worded better, no search term is displayed

Sponsored-by: Catalyst IT
Patch behaves as dexcribed.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>