[Managed.Windows.Forms] Verify that DataMember is valid when setting DataSource

When DataMember is already set before DataSource we need to verify that it refers to a valid property in the DataSource setter, otherwise we'd get an exception in ResetList ().
This fixes the following failing tests: BindingSourceTest.DataMemberBeforeDataSource and BindingSourceTest.DataSourceAssignToDefaultType.

Commit licensed under MIT/X11.
Merge pull request #1117 from akoeplinger/fix-databindingsource

[Managed.Windows.Forms] Verify that DataMember is valid when setting DataSource