[ALOY-250] Alloy should throw errors on XML formatting failures

It seems as though XML formatting errors are being reported in the log, but not aborting the Alloy build process. For example:

<Alloy>
	<Window32 id="index">
		<Button id="b" class="button" onClick="showAlert">Hello, World!</Button>
	</Window>
</Alloy>

This causes the following error message to appear in the log:

end tag name: Window is not match the current start tagName:Window32

But it does not abort the build until a later error forces it to. These XML errors should be caught when they occur and conveyed to the developer so that they can fix them before the app runs, not simply warn them and have them encounter unexpected behavior.