Bug 17375: Search by dateofbirth - handle invalid dates

Bug 17375: Search by dateofbirth - handle invalid dates

Prevent internal software error when searching patron with invalid birth date

To reproduce:

- Go to Home > Patron
- Expand patron search (click on + at the left of the search button)
- In drop down 'Search fields', select 'Date of birth'
- Enter a valid date (e.g. 11.02.1995 if syspref 'dateformat' is set to dmydot)
Result: Search works OK
- Enter an invalid date, e.g. 11.02 or abcd...
Result: Internal server error

- Do a patron search with many results
- Use filter on results screen, select 'Date of birth' as search field and
  enter an invalid date to search (e.g. 'a')
Result: Endless message 'Processing'

To test:
- Apply patch
- Repeat steps above
- In both cases, you should get "No results"

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Lucio Moraes <lmoraes@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>