[ticket/301] Throw Exception on incorrect call of phpbb_fetch_posts

B3P-301
[ticket/301] Cover InvalidArgumentException in phpbb_fetch_posts

B3P-301
[ticket/301] Add higher test coverage of phpbb_fetch_posts

B3P-301
[ticket/301] Extend phpbb_fetch_posts coverage to 100%

B3P-301
Merge pull request #302 from marc1706/ticket/301

[ticket/301] Improve coverage of phpbb_fetch_posts() to 100%