Throwing errors

[RCCoop]

Seeking out errors from AEXMLElement.error always seemed unintuitive to me, where throwing functions are the more idiomatic Swift way to handle this. So here is a pull request that removes the error property, and instead makes the places where existing errors could be created into throwing functions.

This is a breaking change, of course, since it changes the call signature of the following:

• AEXMLDocument.root Getter now throws AEXMLError.rootElementMissing if no root element exists (previously, this would return an empty AEXMLElement with the error stored in its error property).
• AEXMLElement["key"] getter now throws AEXMLError.elementNotFound(String)` if the element with the given name is not found (previously, this would also return an empty element with an error property).

Extra changes I made:

• Updated Unit Tests to Swift Testing
• AEXMLError.elementNotFound now includes the key that wasn't found, so you can chain subscripts together and know which one failed, like so:

// this throws AEXMLError.elementNotFound("badKey1")
let x = try element["goodKey"]["badKey1"]["badKey2"]

Hope you like it. Thanks for creating this library!