Fix page type composer save message

Composer page type form throws adodb mysql exception if always beneath
and no page is selected.

Checks if $page is returned from Page::getByID and sets error message
instead.

http://www.concrete5.org/developers/bugs/5-6-0-2
/mysql-error-on-page-types-composer-setup-when-dont-select-a-page/

Done the concrete5 way
Merge pull request #906 from mkly/bugfix_composer_mysql_error_message

Fix page type composer save message