[ticket/11467] Add language variables for extension exception messages

Remove hard-coded messages and add language variables for extension
exception messages.

PHPBB3-11467
[ticket/11467] Add language variables to tests

Convert the hardcoded strings to language variables.

PHPBB3-11467
[ticket/11467] Add user object to extension manager for tests

PHPBB3-11467
Merge pull request #2447 from n-aleha/ticket/11467

[ticket/11467] Add language variables for extension exception messages

* n-aleha/ticket/11467:
  [ticket/11467] Add user object to extension manager for tests
  [ticket/11467] Add language variables to tests
  [ticket/11467] Add language variables for extension exception messages