Bug 17521: Added missing age limit check

Bug 17521: Added missing age limit check

Following patron modification partial editor had no age constraint
checking:
/cgi-bin/koha/members/memberentry.pl?op=modify&borrowernumber=3&step=3

Test plan:
1) Apply the patch
2) Open profile of a patron
3) Click Edit under "Library use": http://prntscr.com/d1ghim
4) Change category to an invalid one (eg. Adult instead of Kid)
5) Error saying "Patron's age is incorrect for their category." should
be displayed.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Lucio Moraes <lmoraes@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>