Throw exception when using invalid virtual values in DB2

[thiagolvlsantos]

On referenced table name errors throw an exception to make it clear to the user the reference is wrong

i.e.
<table>
   <caption>Users</caption>
.....
</table>

sgbd.cfg.xml
<table alias="Account">
    <column alias="User" table="Users" virtual="true"/>
</table>