Bug 16092: Fix error dialog and introduce the Font Awesome icons when delete a branc...

 Bug 16092: Fix error dialog and introduce the Font Awesome icons when delete a branch category

Also change admin/branch.pl   type => 'error' to type => 'alert' to fit
with wiki <https://wiki.koha-community.org/wiki/Interface_patterns#Errors_and_messages>

To test:
1) Go to Admin -> Libraries and groups
2) Try to delete a library with patrons and bib items
3) See the error message in yellow
4) Try to delete a branch category/group without any library attached
   to it
5) Apply patch and repeat steps 1 to 4. Notice about the changes
6) Test buttons Yes, delete and No, do not delete and verify that they
   work as expected

Signed-off-by: Marc Véron <veron@veron.ch>
NOTE: Patch rebased and reword title according with QA comment 4

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>