[ticket/10945] remove trigger_error call

removes trigger_error call incase no search results are found or start
variable exceeds the amount. So that search query is displayed in the
search box even if no results are returned.

PHPBB3-10945
[ticket/10945] Display search query when search fails in native search

If search fails and common words are present, split_keywords returns false
and search.php file prints appropriate message of common words.
If search fails and there are no common words, split_keywords returns true
and keyword_search returns false so that the search query is displayed in
search box.

PHPBB3-10945
[ticket/10945] Fix return values for split_keywords function

The function now returns false if all the words are common words and true
otherwise.

PHPBB3-10945