Minor change to erro message
Fixes #5371 Make Piwik work PHP session hash function is non default such as sha1 or sha512 or other
id is now VARCHAR field

Tested with: session.hash_function = sha256

Had to use VARCHAR as the session id can be 128 chars eg.
```
$ php -r "var_dump(hash('sha512','test'));"
string(128) "ee26b0dd4af7e749aa1a8ee3c10ae9923f618980772e473f8819a5d4940e0db27ac185f8a0e1d5f84f88bc887fd67b143732c304cc5fa9ad8e6f57f50028a8ff"
```
Merge branch 'master' of github.com:piwik/piwik