Do not use L10n when logging exceptions

In some specific situations, the L10N bundle isn't loadable yet (for
example when there is an issue with the app_config table). In such case,
we still want to be able to log the real exception.

This fixes errors that say "OC_L10N_String::__toString must not throw
exceptions"
Merge pull request #6438 from owncloud/core-logexceptionfix

Do not use L10n when logging exceptions