Make SpriteBank.orig_ctor throw a descriptive error if an element has copy but no path

[SilverDorian46]

SpriteData.Add already throws an error with the message "Sprite '" + xmlElement.Name + "': 'path' is missing!",
but in order to do that, the execution has to enter SpriteData.Add first.

at the moment, if the element has copy, then the execution tries to get the value of path through xmlElement.Attr("path") - which doesn't do a null check - and pass it to SpriteData.Add, which happens before SpriteData.Add itself is executed.
hence, if the element has copy but no path, the program throws a vague NullReferenceException.

this PR patches that for the program to check xmlElement.HasAttr("path"), which is essentially a null check, right before the line where it tries to get xmlElement.Attr("path").
if the check returns false, an exception with the message "Sprite '" + xmlElement.Name + "': 'path' is missing!" is thrown.