Throw error on invalid params

[jmsv]

Throw error when:

• a target key value is not a valid key/string
• .get() function is called with key that isn't indexed
• etc.

[j-m]

On redesign this just returns undefined copying the behaviour of find.

[jmsv]

Feel like an error should be thrown when trying to call get with a key that’s not indexed

[j-m]

Ah I see your point. If options.keys is defined then yes I agree, otherwise no. If the data is dynamic then they might not always have that key but sometimes could, I would rather have undefined or an empty array returned in this case than have to handle an error.