Handle missing relationship methods, string relationship definition

Code changes

• Update docblocks
• Check that the cascade relationship method exists and that it returns the appropriate type
  • A LogicException is now thrown if the method does not exist, as well as if it does exist and does not implement Illuminate\Database\Eloquent\Relations\Relation
• Move iterating over the cascadeDeletes property to a getter method, to ensure the trait functions even without the property defined
• Switch collect to array_filter (minor speed improvement)

Tests

• Add expected message regular expressions for expected exceptions
• Ensure we can accept a string in the cascadeDeletes property
• Ensure we handle relationship methods that don’t exist without triggering a fatal error
• Extract attaching of comments to a private method