Import data of a table without foreign keys show a error.

The error occurs when calling the function `_foreign_key_exists` of the class `abstract_import_manager`

It is an unlikely problem, but occurs if the tables are created without FK (incorrect installation, of course!)
Also will occur if in the future on a new code to import an other table without FK.
Merge pull request #96 from jorix/i-table_no_FK

Import data of a table without foreign keys show a error.