Require selections on Unions, with backwards compat

[rmosolgo]

Fixes #4351

This implementation:

• Warns if an invalid query is encountered and no compat mode is selected;
• Provides a hook for handing invalid queries (which can permit, deny, or provide a custom message)
• Provides a config to reject invalid queries

I think this is a safe default to roll out to anyone already using the gem.

cc @gmac -- finally...