Use "if not exists" when trying to create table #14886

This can avoid some error logs depending on the used database adapter etc. In theory we can then remove catching the error a few lines below but should be fine to keep it as well.

If not exists should be safe to use, was even supported in old versions: http://ftp.nchu.edu.tw/MySQL/doc/refman/4.1/en/create-table.html

Not sure why it wasn't used right away. A comment in the code refers to https://github.com/piwik/piwik/issues/153 but looks like that ID wasn't 100% correctly migrated from the old trac. In any way, using if not exists should be fine...