Refine the error messages on saveObjects

When you pass a single array instead of an array of objects to the `saveObjects` method, the following error is returned:
`PHP Fatal error: Uncaught Algolia\AlgoliaSearch\Exceptions\MissingObjectId:
All objects must have an unique objectID (like a primary key) to be valid.`

The error should be changed to
`PHP Fatal error: Please provide an array of objects instead of a single object.` to better reflect the problem.

See https://secure.helpscout.net/conversation/950642494/173763/