add try/catch around statements that execute intl_is_failure()

Since PHP 5.5.0, the configuration option 'intl.use_exceptions' is available.
When set to true, IntlException's are thrown instead of just setting a error
code, so no calls to intl_is_failure() are necessary.
Without the try/catch, the Int, Float and DateTime I18n validators hard-fail
in case 'intl.use_exceptions' is set to true.

NOTE: catching Exception (super class of IntlException), as IntlException is
not available prior to PHP 5.5.0.

refs #6312
That is actually no problem, ```instanceof``` also works when the class doesn't exist.