[ticket/12826] Add mandatory root namespace indicator to exception catching.

PHPBB3-12826
[ticket/12826] Add optional root namespace indicator for consistency.

PHPBB3-12826
Merge pull request #2812 from bantu/ticket/12826

[ticket/12826] Add root namespace indicator to exception catching.

* bantu/ticket/12826:
  [ticket/12826] Add optional root namespace indicator for consistency.
  [ticket/12826] Add mandatory root namespace indicator to exception catching.