Bug 18674: TZ error handling

Bug 18674: TZ error handling

This patch adds C4::Context->timezone bad timezone handling.
The calculated 'effective' timezone is tested with the right tool and a
fallback to 'local' is added. A warning is printed in the logs.

A test for this is added to about.pl too, along with the right warning
messages in case of problems.

Tests are added for both invalid TZ and to make sure the warning is
raised.

To test:
- Apply the patch
- Run:
  $ kshell
 k$ prove t/timezones.t
=> SUCCESS: All tests pass

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>