Bug 20386: Improve warning and error messages for Search Engine Configuration

Bug 20386: Improve warning and error messages for Search Engine Configuration

This patch updates a warning message and error message in the Search
engine configuration for Elasticsearch.

Note: elasticsearch does not need to be active in order to test.

To test warning message:
 1. go to: cgi-bin/koha/admin/searchengine/elasticsearch/mappings.pl
 2. note the warning message
 3. Apply patch
 4. Note the change in warning message. Confirm it makes sense.

To test error message:
  Apply patch and check the changed message in the template file mappings.tt and confirm it makes sense.

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>