Fix issue where exceptions triggered by extensions could be silently swallowed

[kinglozzer]

Something I overlooked in #229. Logging::getLogger() uses the default logging service, SilverStripe\Omnipay\Logger. If that has been set to a NullLogger as per the instructions in added #229, then exceptions thrown during safeExtend() can be swallowed in live mode.

This change ensures that if a separate logger is set up for fatal errors (as described in #229), it will be used by safeExtend() and safeguard() so that exceptions can still be logged. As per the other pull request, if no separate Injector service has been set up for SilverStripe\Omnipay\ExceptionLogger then it inherits from SilverStripe\Omnipay\Logger instead:

silverstripe-omnipay/_config/logging.yml

Line 7 in a44ee69

SilverStripe\Omnipay\ExceptionLogger: '%$SilverStripe\Omnipay\Logger'

I’ve pushed a 3.1 branch for this, so it can be tagged as 3.1.2 without including the other changes that are currently in master.

[kinglozzer]

Will self-merge this soon if no one objects. Out of the box things should be fine, but it’s a potentially nasty bug if someone has opted into this behaviour https://github.com/silverstripe/silverstripe-omnipay/blob/master/docs/en/Logging.md#disable-non-fatal-error-logging as it could mean that fatal errors during onPaymentComplete() handlers may not be logged