Improve error message for obtaining a JDBC connection

[DavideD]

When the application tries to access a JDBC connection, we throw the following error:

Caused by: java.sql.SQLException: Not using JDBC

This happens in NoJdbcConnectionProvider#getConnection.

When this happens, it means that we haven't tested a particular mapping or configuration. In a perfect world, a user should never see this error.

We should think of a better error message to explain the user that their use case is currently non supported.
And it makes sense to add the error in the logger.

[yrodiere]

+1 it does seem like a quick way to massively improve the experience of new users.