added some nicer error message when a provider is not found

Now looks like this, which should help troubleshooting provider issues

Unable to find the requested .Net Framework Data Provider.  It may not be installed.

I did find these Factories:|System.Data.Odbc||System.Data.OleDb||System.Data.OracleClient||System.Data.SqlClient||System.Data.SqlServerCe.4.0|