Bug: crash on invalid relationship name

[bart-degreed]

When getting a misspelled relationship name, the code crashes at CurrentRequestMiddleware.GetCurrentEntity() due to recent changes.

Repro steps:

• Run the unit test Can_Get_TodoItem_WithOwner to have a record in database
• Start project JsonApiDotNetCoreExample
• GET http://localhost:1479/api/v1/todoItems/1?include=owner and see todoItems resource followed by included owner
• GET http://localhost:1479/api/v1/todoItems/1/owner and observe successful response with person
• GET http://localhost:1479/api/v1/todoItems/1/ownerX and observe HTTP 500