Bug 21221: Shortcut memberentry scripts if patron does not exist

Bug 21221: Shortcut memberentry scripts if patron does not exist

If borrowernumber is passed and that it does not refer to a valid patron
in DB, we should not continue the script and display an error instead.

Test plan:
Create a patron
Edit a patron
=> Both should work ok
You can also test the other action memberentry.pl manage.

Edit it again but modify the borrowernumber parameter
=> You should see a friendly user message saying that the patron does
not exist.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>