Throw an error if a model's $primaryKey is an array

[andrechalom]

There are lots of closed issues regarding the use of composite primary keys, and all of them have been closed by the dev team (eg. laravel/framework#5517). However, the documentation does not clearly mention that composite keys are not supported, and even has an example for generating a migration with a composite primary key.

I understand the reason why this feature will not be implemented, but can you provide a check to provide the application developers with a clear error when they try to implement a composite primary? As it stands, when I tried to use a composite key in one of my models everything started to fail with an "illegal offset type" error, which is not helpful at all.

(Migrated from laravel/framework#20393)

[taylorotwell]

No plans on changing this for now.