Improve error formatting for validating metadata in commerce-kit

[typeofweb]

Currently, metadata for different entities is validated in runtime. In case there are any issues, a Zod error is thrown.

Zod errors are useful but not instantly readable to our users. This should be improved. Moreover, sometimes context information is lacking – i.e., which product validation did fail?

• keep as much information in errors as possible
• consider using .flatten() from zod to group errors differently
• consider throwing a custom error with all the information from Zod + more context (i.e., entity ID)