Bug 35477: Show a warning when non-existent items are added to the waiting list

Bug 35477: Show a warning when non-existent items are added to the waiting list

Only messages "X new items added" or "No items added" were displayed.
With this patch we will display:
A warning with "No items added"
A warning with "X new items added. Y items not found." when some items
have not been added
A messages "X new items added" when everything went well

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit ed37c7e68d350a78b30080b2b4673083a471c50e)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>