Bundle key and Bundle are required if you are mapping to a non-reference field

Problem/Motivation

Like #3001828: field type integeris not a recognized reference type, or:How to entity_lookup a commerce_product I'm mapping to an integer field, but I don't want to restrict by bundle. In fact in one case there isn't a bundle type fields (it's not a bundleable entity). It shouldn't be a exception if the bundle isn't present, as it's not actually required.

Steps to reproduce

As per linked example, but without bundle_type and bundle.

Proposed resolution

Don't throw an exception if there is only the bundle type or bundle missing. Still throw an exception if the entity_type or value field are missing because those really are required.

Remaining tasks

Patch.

User interface changes

None.

API changes

None except you can uses a non-reference field without a bundle type.

Data model changes

None.